chore(frontend): target ES2022 (#39420)

Bump the TypeScript and Vite targets from ES2020 to ES2022. The frontend
already calls ES2022 APIs like `Object.hasOwn` and `Array.prototype.at`
without polyfills, so downleveling class syntax only added helpers. JS
output shrinks by 36 KB.

The only new syntax-level requirement is Safari 16.4 for mermaid, which
ships class static blocks. Pale Moon supports all emitted syntax since
32.2.

The ES2022 target implies `useDefineForClassFields`, so
`noImplicitOverride` is enabled to flag class fields that shadow
inherited DOM properties.

tools/eslint-rules/unescaped-html-literal.test.ts

@@ -4,13 +4,7 @@
 import rule from './unescaped-html-literal.ts';
 import {RuleTester} from 'eslint';
 
-class VitestRuleTester extends RuleTester {
-  static describe = describe;
-  static it = it;
-  static itOnly = it.only;
-}
-
-const ruleTester = new VitestRuleTester();
+const ruleTester = new RuleTester();
 
 ruleTester.run('unescaped-html-literal', rule, {
   valid: [

tsconfig.json

@@ -10,7 +10,7 @@
     "${configDir}/web_src/js/**/.*",
   ],
   "compilerOptions": {
-    "target": "es2020",
+    "target": "es2022",
     "module": "esnext",
     "moduleResolution": "bundler",
     "lib": ["dom", "esnext", "webworker"],
@@ -26,6 +26,7 @@
     "libReplacement": false,
     "noEmit": true,
     "noFallthroughCasesInSwitch": true,
+    "noImplicitOverride": true,
     "noImplicitReturns": true,
     "noUncheckedSideEffectImports": true,
     "noPropertyAccessFromIndexSignature": false,

vite.config.ts

@@ -61,7 +61,7 @@ function commonViteOpts({build, ...other}: InlineConfig): InlineConfig {
       outDir,
       emptyOutDir: false,
       sourcemap: enableSourcemap !== 'false',
-      target: 'es2020',
+      target: 'es2022',
       minify: isProduction ? 'oxc' : false,
       cssMinify: isProduction ? 'esbuild' : false,
       chunkSizeWarningLimit: Infinity,

web_src/js/webcomponents/relative-time.ts

@@ -319,11 +319,11 @@ class RelativeTime extends HTMLElement {
     return undefined;
   }
 
-  get prefix(): string {
+  override get prefix(): string {
     return this.getAttribute('prefix') ?? (this.format === 'datetime' ? '' : 'on');
   }
 
-  set prefix(v: string) {
+  override set prefix(v: string) {
     this.setAttribute('prefix', v);
   }