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/utils/match.test.ts

@@ -0,0 +1,50 @@
+import {matchEmoji, matchMention} from './match.ts';
+
+test('matchEmoji', () => {
+  expect(matchEmoji('')).toEqual([
+    '+1',
+    '-1',
+    '100',
+    '1234',
+    '1st_place_medal',
+    '2nd_place_medal',
+  ]);
+
+  expect(matchEmoji('hea')).toEqual([
+    'headphones',
+    'headstone',
+    'health_worker',
+    'hear_no_evil',
+    'heard_mcdonald_islands',
+    'heart',
+  ]);
+
+  expect(matchEmoji('hear')).toEqual([
+    'hear_no_evil',
+    'heard_mcdonald_islands',
+    'heart',
+    'heart_decoration',
+    'heart_eyes',
+    'heart_eyes_cat',
+  ]);
+
+  expect(matchEmoji('poo')).toEqual([
+    'poodle',
+    'hankey',
+    'spoon',
+    'bowl_with_spoon',
+  ]);
+
+  expect(matchEmoji('1st_')).toEqual([
+    '1st_place_medal',
+  ]);
+
+  expect(matchEmoji('jellyfis')).toEqual([
+    'jellyfish',
+  ]);
+});
+
+test('matchMention', () => {
+  expect(matchMention('')).toEqual(window.config.mentionValues.slice(0, 6));
+  expect(matchMention('user4')).toEqual([window.config.mentionValues[3]]);
+});