Extensions: Add regex engine

Regex is a method that is commonly used to find and replace parts
of a string using a single pattern. Add support for using regex in
SillyTavern which allows users to dynamically change various aspects
of the chatting experience.

Users are able to choose where a given regex script should apply
(both invasive and non-invasive options!). Invasive options alter
chat history while non-invasive alters markdown display for the
entire chat.

A new variable called {{match}} is added in regex scripts which
substitutes in the found match from the original find regex script.

There is a lot more that can be added to this extension, but for now,
this is enough.

Signed-off-by: kingbri <bdashore3@proton.me>

public/scripts/extensions/regex/engine.js

@@ -0,0 +1,58 @@
+import { substituteParams } from "../../../script.js";
+export {
+    runRegexScript
+}
+
+// From: https://github.com/IonicaBizau/regex-parser.js/blob/master/lib/index.js
+function regexFromString(input) {
+    // Parse input
+    var m = input.match(/(\/?)(.+)\1([a-z]*)/i);
+
+    // Invalid flags
+    if (m[3] && !/^(?!.*?(.).*?\1)[gmixXsuUAJ]+$/.test(m[3])) {
+        return RegExp(input);
+    }
+
+    // Create the regular expression
+    return new RegExp(m[2], m[3]);
+}
+
+// Runs the provided regex script on the given string
+function runRegexScript(regexScript, rawString) {
+    if (!!(regexScript.disabled)) {
+        return;
+    }
+
+    let match;
+    let newString;
+    const findRegex = regexFromString(regexScript.findRegex);
+    while ((match = findRegex.exec(rawString)) !== null) {
+        const fencedMatch = match[0];
+        const capturedMatch = match[1];
+
+        // TODO: Use substrings for replacement. But not necessary at this time.
+        // A substring is from match.index to match.index + match[0].length or fencedMatch.length
+        const subReplaceString = substituteRegexParams(regexScript.replaceString, { regexMatch: capturedMatch ?? fencedMatch });
+        if (!newString) {
+            newString = rawString.replace(fencedMatch, subReplaceString);
+        } else {
+            newString = newString.replace(fencedMatch, subReplaceString);
+        }
+
+        // If the regex isn't global, break out of the loop
+        if (!findRegex.flags.includes('g')) {
+            break;
+        }
+    }
+
+    return newString;
+}
+
+// Substitutes parameters 
+function substituteRegexParams(rawString, { regexMatch }) {
+    let finalString = rawString;
+    finalString = finalString.replace("{{match}}", regexMatch);
+    finalString = substituteParams(finalString);
+
+    return finalString;
+}