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/markup/anchors.ts

@@ -1,4 +1,4 @@
-import {svg} from '../svg.js';
+import {svg} from '../svg.ts';
 
 const addPrefix = (str) => `user-content-${str}`;
 const removePrefix = (str) => str.replace(/^user-content-/, '');

web_src/js/markup/codecopy.ts

@@ -1,4 +1,4 @@
-import {svg} from '../svg.js';
+import {svg} from '../svg.ts';
 
 export function makeCodeCopyButton() {
   const button = document.createElement('button');

web_src/js/markup/content.ts

@@ -1,8 +1,8 @@
-import {renderMermaid} from './mermaid.js';
-import {renderMath} from './math.js';
-import {renderCodeCopy} from './codecopy.js';
-import {renderAsciicast} from './asciicast.js';
-import {initMarkupTasklist} from './tasklist.js';
+import {renderMermaid} from './mermaid.ts';
+import {renderMath} from './math.ts';
+import {renderCodeCopy} from './codecopy.ts';
+import {renderAsciicast} from './asciicast.ts';
+import {initMarkupTasklist} from './tasklist.ts';
 
 // code that runs for all markup content
 export function initMarkupContent() {

web_src/js/markup/math.ts

@@ -1,4 +1,4 @@
-import {displayError} from './common.js';
+import {displayError} from './common.ts';
 
 function targetElement(el) {
   // The target element is either the current element if it has the

web_src/js/markup/mermaid.ts

@@ -1,6 +1,6 @@
-import {isDarkTheme} from '../utils.js';
-import {makeCodeCopyButton} from './codecopy.js';
-import {displayError} from './common.js';
+import {isDarkTheme} from '../utils.ts';
+import {makeCodeCopyButton} from './codecopy.ts';
+import {displayError} from './common.ts';
 
 const {mermaidMaxSourceCharacters} = window.config;
 

web_src/js/markup/tasklist.ts

@@ -1,5 +1,5 @@
-import {POST} from '../modules/fetch.js';
-import {showErrorToast} from '../modules/toast.js';
+import {POST} from '../modules/fetch.ts';
+import {showErrorToast} from '../modules/toast.ts';
 
 const preventListener = (e) => e.preventDefault();