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/admin/common.ts

@@ -1,7 +1,7 @@
 import $ from 'jquery';
-import {checkAppUrl} from '../common-page.js';
-import {hideElem, showElem, toggleElem} from '../../utils/dom.js';
-import {POST} from '../../modules/fetch.js';
+import {checkAppUrl} from '../common-page.ts';
+import {hideElem, showElem, toggleElem} from '../../utils/dom.ts';
+import {POST} from '../../modules/fetch.ts';
 
 const {appSubUrl} = window.config;
 

web_src/js/features/admin/config.ts

@@ -1,5 +1,5 @@
-import {showTemporaryTooltip} from '../../modules/tippy.js';
-import {POST} from '../../modules/fetch.js';
+import {showTemporaryTooltip} from '../../modules/tippy.ts';
+import {POST} from '../../modules/fetch.ts';
 
 const {appSubUrl} = window.config;
 

web_src/js/features/admin/selfcheck.ts

@@ -1,5 +1,5 @@
-import {toggleElem} from '../../utils/dom.js';
-import {POST} from '../../modules/fetch.js';
+import {toggleElem} from '../../utils/dom.ts';
+import {POST} from '../../modules/fetch.ts';
 
 const {appSubUrl} = window.config;