Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/features/emoji.ts

@@ -0,0 +1,38 @@
+import emojis from '../../../assets/emoji.json';
+
+const {assetUrlPrefix, customEmojis} = window.config;
+
+const tempMap = {...customEmojis};
+for (const {emoji, aliases} of emojis) {
+  for (const alias of aliases || []) {
+    tempMap[alias] = emoji;
+  }
+}
+
+export const emojiKeys = Object.keys(tempMap).sort((a, b) => {
+  if (a === '+1' || a === '-1') return -1;
+  if (b === '+1' || b === '-1') return 1;
+  return a.localeCompare(b);
+});
+
+const emojiMap = {};
+for (const key of emojiKeys) {
+  emojiMap[key] = tempMap[key];
+}
+
+// retrieve HTML for given emoji name
+export function emojiHTML(name) {
+  let inner;
+  if (Object.hasOwn(customEmojis, name)) {
+    inner = `<img alt=":${name}:" src="${assetUrlPrefix}/img/emoji/${name}.png">`;
+  } else {
+    inner = emojiString(name);
+  }
+
+  return `<span class="emoji" title=":${name}:">${inner}</span>`;
+}
+
+// retrieve string for given emoji name
+export function emojiString(name) {
+  return emojiMap[name] || `:${name}:`;
+}