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
https://github.com/go-gitea/gitea/pull/31559/commits/61105d0618e285d97e95044bfb64415f364a4526
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/url.test.ts

@@ -0,0 +1,13 @@
+import {pathEscapeSegments, isUrl} from './url.ts';
+
+test('pathEscapeSegments', () => {
+  expect(pathEscapeSegments('a/b/c')).toEqual('a/b/c');
+  expect(pathEscapeSegments('a/b/ c')).toEqual('a/b/%20c');
+});
+
+test('isUrl', () => {
+  expect(isUrl('https://example.com')).toEqual(true);
+  expect(isUrl('https://example.com/')).toEqual(true);
+  expect(isUrl('https://example.com/index.html')).toEqual(true);
+  expect(isUrl('/index.html')).toEqual(false);
+});