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/webcomponents/absolute-date.ts

@@ -0,0 +1,40 @@
+import {Temporal} from 'temporal-polyfill';
+
+export function toAbsoluteLocaleDate(dateStr, lang, opts) {
+  return Temporal.PlainDate.from(dateStr).toLocaleString(lang ?? [], opts);
+}
+
+window.customElements.define('absolute-date', class extends HTMLElement {
+  static observedAttributes = ['date', 'year', 'month', 'weekday', 'day'];
+
+  update = () => {
+    const year = this.getAttribute('year') ?? '';
+    const month = this.getAttribute('month') ?? '';
+    const weekday = this.getAttribute('weekday') ?? '';
+    const day = this.getAttribute('day') ?? '';
+    const lang = this.closest('[lang]')?.getAttribute('lang') ||
+      this.ownerDocument.documentElement.getAttribute('lang') || '';
+
+    // only use the first 10 characters, e.g. the `yyyy-mm-dd` part
+    const dateStr = this.getAttribute('date').substring(0, 10);
+
+    if (!this.shadowRoot) this.attachShadow({mode: 'open'});
+    this.shadowRoot.textContent = toAbsoluteLocaleDate(dateStr, lang, {
+      ...(year && {year}),
+      ...(month && {month}),
+      ...(weekday && {weekday}),
+      ...(day && {day}),
+    });
+  };
+
+  attributeChangedCallback(_name, oldValue, newValue) {
+    if (!this.initialized || oldValue === newValue) return;
+    this.update();
+  }
+
+  connectedCallback() {
+    this.initialized = false;
+    this.update();
+    this.initialized = true;
+  }
+});