URLらしき文字列なのにリンク(aタグ）になっていないものを対象に、順次リンクを付与していきます。

convertTextToLink.user.js

// ==UserScript==
// @name        リンク化
// @namespace   jp.hateblo.htsign
// @version     1.2.3
// @grant       none
// @downloadURL https://gist.github.com/htsign/5eed5473a9e75c7c45f3a5571d7d0803/raw/convertTextToLink.user.js
// ==/UserScript==

(() => {
    
    class ElementFactory {
        constructor(tagName, attrs) {
            this.template = document.createElement(tagName);
            for (let {attr, value} of attrs) this.template[attr] = value;
        }
        
        create(attrs) {
            const elem = this.template.cloneNode(true);
            for (let {attr, value} of attrs) elem[attr] = value;
            return elem;
        }
    }
    
    
    const pattern = /(?:h?t?tp(s?):\/\/(?:[\w-]+|[^ -~｡-ﾟ]+)\.[a-zA-Z]{2,4}[^\s　"']*)|(?:[^/\s"'(（「【『]*?(?:[\w-]+|[^ -~｡-ﾟ]+)\.(?:com|org|net|edu|gov|jp|to|tv|fm|info|(?:co|or|ne|ac|go)\.(?:jp|uk|fr|de))[^\s　"')）」】』]*)/g;
    const anchorFactory = new ElementFactory("a", [{attr: "target", value: "_blank"}]);
    
    const observer = new MutationObserver(records => records.forEach(r => setTimeout(convertTextToLink, 10, r.target)));
    observer.observe(document.body, {childList: true});
    
    window.addEventListener("unload", _ => observer.disconnect());
    convertTextToLink(document.body);
    
    
    function convertTextToLink(rootNode) {
        const filter = node => {
            // 祖先ノードの中に a, script があれば true
            const isAnchorOrScript = function recur(node) {
                const name = node.nodeName.toLowerCase();
                if (node instanceof Element && ["a", "script"].includes(name)) return true;
                return node.parentNode ? recur(node.parentNode) : false;
            }(node);
            return isAnchorOrScript ? NodeFilter.FILTER_SKIP : NodeFilter.FILTER_ACCEPT;
        };
        const iter  = document.createNodeIterator(rootNode, NodeFilter.SHOW_TEXT, filter, false);
        const range = document.createRange();
        
        let current;
        while ((current = iter.nextNode()) !== null) {
            const getPathName = url => {
                const pattern2 = /:\/\//g;
                return pattern2.exec(url) ? url.slice(pattern2.lastIndex) : url;
            };
            let regRes, matchStack = [];
            
            // マッチがなくなるまでループしてスタック
            while ((regRes = pattern.exec(current.data)) !== null) {
                matchStack.push({
                    url   : `http${regRes[1] || ""}://${getPathName(regRes[0])}`,
                    start : regRes.index,
                    end   : pattern.lastIndex,
                });
            }
            // 後ろから1つずつ取り出して変更を適用
            while (matchStack.length) {
                const item = matchStack.pop();
                const a = anchorFactory.create([{attr: "href", value: item.url}]);
                
                range.setStart (current, item.start);
                range.setEnd   (current, item.end  );
                range.surroundContents(a);
            }
        }
        range.detach();
    }
    
}).call(this);