You can paste this into a bookmarklet:

javascript:(function(){var l=document.getElementsByTagName('a');for(var i=0; i<l.length; i++){if(l[i].href.indexOf('&page=')==-1){l[i].target='_blank';}}window.onscroll = function(){var l=document.getElementsByTagName('a');for(var i=0; i<l.length; i++){if(l[i].href.indexOf('&page=')==-1){l[i].target='_blank';}}};})();

Or if you use Tampermonkey, here is the full userscript:

// ==UserScript==
// @name         old.lemmy.world - Open Links in New Tab/Window
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Emulates Reddit's behavior when "open in new tab" is checked.
// @author       /u/Supervisor194
// @match        https://old.lemmy.world/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=tampermonkey.net
// @grant        none
// ==/UserScript==

(function() {
    // Your code here...
    var l=document.getElementsByTagName('a');
    for(var i=0; i<l.length; i++){
        if(l[i].href.indexOf('&page=')==-1){
            l[i].target='_blank';
        }
    }
    window.onscroll = function(){
        var l=document.getElementsByTagName('a');
        for(var i=0; i<l.length; i++){
            if(l[i].href.indexOf('&page=')==-1){
                l[i].target='_blank';
            }
        }
    }
})();