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/svg.test.ts

@@ -0,0 +1,27 @@
+import {svg, SvgIcon, svgParseOuterInner} from './svg.ts';
+import {createApp, h} from 'vue';
+
+test('svg', () => {
+  expect(svg('octicon-repo')).toMatch(/^<svg/);
+  expect(svg('octicon-repo', 16)).toContain('width="16"');
+  expect(svg('octicon-repo', 32)).toContain('width="32"');
+});
+
+test('svgParseOuterInner', () => {
+  const {svgOuter, svgInnerHtml} = svgParseOuterInner('octicon-repo');
+  expect(svgOuter.nodeName).toMatch('svg');
+  expect(svgOuter.classList.contains('octicon-repo')).toBeTruthy();
+  expect(svgInnerHtml).toContain('<path');
+});
+
+test('SvgIcon', () => {
+  const root = document.createElement('div');
+  createApp({render: () => h(SvgIcon, {name: 'octicon-link', size: 24, class: 'base', className: 'extra'})}).mount(root);
+  const node = root.firstChild;
+  expect(node.nodeName).toEqual('svg');
+  expect(node.getAttribute('width')).toEqual('24');
+  expect(node.getAttribute('height')).toEqual('24');
+  expect(node.classList.contains('octicon-link')).toBeTruthy();
+  expect(node.classList.contains('base')).toBeTruthy();
+  expect(node.classList.contains('extra')).toBeTruthy();
+});