Markdown: Add ability to exclude specific strings

A comma-separated list of markdown strings provided by the user can
be excluded as needed. This is combined with the set chat separator
to provide a seamless experience when chatting.

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

public/scripts/showdown-exclusion.js

@@ -0,0 +1,35 @@
+import { power_user } from './power-user.js';
+
+// Showdown extension to make chat separators (dinkuses) ignore markdown formatting
+export const markdownExclusionExt = () => {
+    if (!power_user) {
+        console.log("Showdown-dinkus extension: power_user wasn't found! Returning.");
+        return []
+    }
+
+    let combinedExcludeString = '';
+    if (power_user.custom_chat_separator) {
+        combinedExcludeString += `${power_user.custom_chat_separator},`;
+    }
+
+    if (power_user.markdown_escape_strings) {
+        combinedExcludeString += power_user.markdown_escape_strings;
+    }
+
+    const escapedExclusions = combinedExcludeString
+        .split(",")
+        .map((element) => `(${element.split('').map((char) => `\\${char}`).join('')})`);
+
+
+    // No exclusions? No extension!
+    if (!combinedExcludeString || combinedExcludeString.length === 0 || escapedExclusions.length === 0) {
+        return [];
+    }
+
+    const replaceRegex = new RegExp(`^(${escapedExclusions.join("|")})\n`, "gm");
+    return [{
+        type: "lang",
+        regex: replaceRegex,
+        replace: ((match) => match.replace(replaceRegex, `\u0000${match} \n`))
+    }];
+}