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/time.ts

@@ -0,0 +1,72 @@
+import dayjs from 'dayjs';
+import utc from 'dayjs/plugin/utc.js';
+import {getCurrentLocale} from '../utils.ts';
+
+dayjs.extend(utc);
+
+/**
+ * Returns an array of millisecond-timestamps of start-of-week days (Sundays)
+ *
+ * @param startConfig The start date. Can take any type that `Date` accepts.
+ * @param endConfig The end date. Can take any type that `Date` accepts.
+ */
+export function startDaysBetween(startDate, endDate) {
+  const start = dayjs.utc(startDate);
+  const end = dayjs.utc(endDate);
+
+  let current = start;
+
+  // Ensure the start date is a Sunday
+  while (current.day() !== 0) {
+    current = current.add(1, 'day');
+  }
+
+  const startDays = [];
+  while (current.isBefore(end)) {
+    startDays.push(current.valueOf());
+    current = current.add(1, 'week');
+  }
+
+  return startDays;
+}
+
+export function firstStartDateAfterDate(inputDate) {
+  if (!(inputDate instanceof Date)) {
+    throw new Error('Invalid date');
+  }
+  const dayOfWeek = inputDate.getUTCDay();
+  const daysUntilSunday = 7 - dayOfWeek;
+  const resultDate = new Date(inputDate.getTime());
+  resultDate.setUTCDate(resultDate.getUTCDate() + daysUntilSunday);
+  return resultDate.valueOf();
+}
+
+export function fillEmptyStartDaysWithZeroes(startDays, data) {
+  const result = {};
+
+  for (const startDay of startDays) {
+    result[startDay] = data[startDay] || {'week': startDay, 'additions': 0, 'deletions': 0, 'commits': 0};
+  }
+
+  return Object.values(result);
+}
+
+let dateFormat;
+
+// format a Date object to document's locale, but with 24h format from user's current locale because this
+// option is a personal preference of the user, not something that the document's locale should dictate.
+export function formatDatetime(date) {
+  if (!dateFormat) {
+    // TODO: replace `hour12` with `Intl.Locale.prototype.getHourCycles` once there is broad browser support
+    dateFormat = new Intl.DateTimeFormat(getCurrentLocale(), {
+      day: 'numeric',
+      month: 'short',
+      year: 'numeric',
+      hour: 'numeric',
+      hour12: !Number.isInteger(Number(new Intl.DateTimeFormat([], {hour: 'numeric'}).format())),
+      minute: '2-digit',
+      timeZoneName: 'short',
+    });
+  }
+  return dateFormat.format(date);
+}