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/markup/content.ts

@@ -0,0 +1,18 @@
+import {renderMermaid} from './mermaid.ts';
+import {renderMath} from './math.ts';
+import {renderCodeCopy} from './codecopy.ts';
+import {renderAsciicast} from './asciicast.ts';
+import {initMarkupTasklist} from './tasklist.ts';
+
+// code that runs for all markup content
+export function initMarkupContent() {
+  renderMermaid();
+  renderMath();
+  renderCodeCopy();
+  renderAsciicast();
+}
+
+// code that only runs for comments
+export function initCommentContent() {
+  initMarkupTasklist();
+}