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/repo-issue-pr-form.ts

@@ -0,0 +1,10 @@
+import {createApp} from 'vue';
+import PullRequestMergeForm from '../components/PullRequestMergeForm.vue';
+
+export function initRepoPullRequestMergeForm() {
+  const el = document.querySelector('#pull-request-merge-form');
+  if (!el) return;
+
+  const view = createApp(PullRequestMergeForm);
+  view.mount(el);
+}