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/contextpopup.js

@@ -1,45 +0,0 @@
-import {createApp} from 'vue';
-import ContextPopup from '../components/ContextPopup.vue';
-import {parseIssueHref} from '../utils.js';
-import {createTippy} from '../modules/tippy.js';
-
-export function initContextPopups() {
-  const refIssues = document.querySelectorAll('.ref-issue');
-  attachRefIssueContextPopup(refIssues);
-}
-
-export function attachRefIssueContextPopup(refIssues) {
-  for (const refIssue of refIssues) {
-    if (refIssue.classList.contains('ref-external-issue')) {
-      return;
-    }
-
-    const {owner, repo, index} = parseIssueHref(refIssue.getAttribute('href'));
-    if (!owner) return;
-
-    const el = document.createElement('div');
-    el.classList.add('tw-p-3');
-    refIssue.parentNode.insertBefore(el, refIssue.nextSibling);
-
-    const view = createApp(ContextPopup);
-
-    try {
-      view.mount(el);
-    } catch (err) {
-      console.error(err);
-      el.textContent = 'ContextPopup failed to load';
-    }
-
-    createTippy(refIssue, {
-      theme: 'default',
-      content: el,
-      placement: 'top-start',
-      interactive: true,
-      role: 'dialog',
-      interactiveBorder: 5,
-      onShow: () => {
-        el.firstChild.dispatchEvent(new CustomEvent('ce-load-context-popup', {detail: {owner, repo, index}}));
-      },
-    });
-  }
-}