Spoiler for How to create a Config Plugin:

1. Create a new project
2. Add scape-editor.jar as a library to that project
3. Define a class like this

Code:
package plugin;

import scape.editor.api.io.RSBuffer;
import scape.editor.gui.plugin.Plugin;
import scape.editor.gui.plugin.extension.config.NpcDefinitionExtension;

@Plugin(name="OSDC Npc Definition Plugin", authors = "Nshusa", version = 1.0)
public class NpcDefinitionPlugin extends NpcDefinitionExtension {

    @Override
    protected String fileName() {
        return "npc.dat";
    }

    @Override
    protected void decode(int currentIndex, RSBuffer buffer) {
        id = currentIndex;
        while(true) {
            int opcode = buffer.readUByte();
            if (opcode == 0) {
                return;
            } else if (opcode == 1) {
                int len = buffer.readUByte();
                modelId = new int[len];
                for (int i = 0; i < len; i++) {
                    modelId[i] = buffer.readUShort();
                }
            } else if (opcode == 2) {
                name = buffer.readString();
            } else if (opcode == 12) {
                size = buffer.readUByte();
            } else if (opcode == 13) {
                standingAnimation = buffer.readUShort();
            } else if (opcode == 14) {
                walkingAnimation = buffer.readUShort();
            } else if (opcode == 15) {
                buffer.readUShort();
            } else if (opcode == 16) {
                buffer.readUShort();
            } else if (opcode == 17) {
                walkingAnimation = buffer.readUShort();
                halfTurnAnimation = buffer.readUShort();
                quarterClockwiseTurnAnimation = buffer.readUShort();
                quarterAnticlockwiseTurnAnimation = buffer.readUShort();
                if (halfTurnAnimation == 65535) {
                    halfTurnAnimation = walkingAnimation;
                }
                if (quarterClockwiseTurnAnimation == 65535) {
                    quarterClockwiseTurnAnimation = walkingAnimation;
                }
                if (quarterAnticlockwiseTurnAnimation == 65535) {
                    quarterAnticlockwiseTurnAnimation = walkingAnimation;
                }
            } else if (opcode >= 30 && opcode < 35) {
                if (actions == null) {
                    actions = new String[5];
                }

                actions[opcode - 30] = buffer.readString();

                if (actions[opcode - 30].equalsIgnoreCase("Hidden")) {
                    actions[opcode - 30] = null;
                }
            } else if (opcode == 40) {
                int len = buffer.readUByte();
                recolorOriginal = new int[len];
                recolorTarget = new int[len];
                for (int i = 0; i < len; i++) {
                    recolorOriginal[i] = buffer.readUShort();
                    recolorTarget[i] = buffer.readUShort();
                }

            } else if (opcode == 41) {
                int len = buffer.readUByte();

                for (int i = 0; i < len; i++) {
                    buffer.readUShort(); // textures
                    buffer.readUShort();
                }
            } else if (opcode == 60) {
                int len = buffer.readUByte();
                additionalModels = new int[len];
                for (int i = 0; i < len; i++) {
                    additionalModels[i] = buffer.readUShort();
                }
            } else if (opcode == 93) {
                aBoolean87 = false;
            } else if (opcode == 95)
                combatLevel = buffer.readUShort();
            else if (opcode == 97)
                scaleXZ = buffer.readUShort();
            else if (opcode == 98)
                scaleY = buffer.readUShort();
            else if (opcode == 99)
                aBoolean93 = true;
            else if (opcode == 100)
                lightModifier = buffer.readByte();
            else if (opcode == 101)
                shadowModifier = buffer.readByte();
            else if (opcode == 102)
                headIcon = buffer.readUShort();
            else if (opcode == 103)
                rotation = buffer.readUShort();
            else if (opcode == 106 || opcode == 118) {
                varbit = buffer.readUShort();

                if (varbit == 65535) {
                    varbit = -1;
                }

                varp = buffer.readUShort();

                if (varp == 65535) {
                    varp = -1;
                }

                int value = -1;

                if (opcode == 118) {
                    value = buffer.readUShort();
                }

                int len = buffer.readUByte();
                morphisms = new int[len + 2];
                for (int i = 0; i <= len; i++) {
                    morphisms[i] = buffer.readUShort();
                    if (morphisms[i] == 65535) {
                        morphisms[i] = -1;
                    }
                }
                morphisms[len + 1] = value;
            } else if (opcode == 109) {
                aBoolean84 = false;
            } else if (opcode == 107 || opcode == 111) {

            } else {
                System.out.println(String.format("npc def invalid opcode: %d", opcode));
            }
        }
    }

    public int id = -1;
    public int quarterAnticlockwiseTurnAnimation = -1;
    public int varbit = -1;
    public int halfTurnAnimation = -1;
    public int varp = -1;
    public int combatLevel = -1;
    public final int anInt64 = 1834;
    public String name;
    public String actions[];
    public int walkingAnimation = -1;
    public int size = 1;
    public int[] recolorTarget;
    public int[] additionalModels;
    public int headIcon = -1;
    public int[] recolorOriginal;
    public int standingAnimation = -1;
    public long interfaceType = -1L;
    public int rotation = 32;
    public int quarterClockwiseTurnAnimation = -1;
    public boolean aBoolean84 = true;
    public int lightModifier;
    public int scaleY = 128;
    public boolean aBoolean87 = true;
    public int morphisms[];
    public int scaleXZ = 128;
    public int shadowModifier;
    public boolean aBoolean93;
    public int[] modelId;
    public int interfaceZoom = 0;

}