Upgrade ESLint to v9 with flat config (#9680)

* Upgrade ESLint to v9 with flat config

- Upgrade eslint from v8 to v9.18.0
- Migrate from .eslintrc.js to eslint.config.mjs (flat config)
- Upgrade typescript-eslint to v8.20.0 (unified package)
- Replace eslint-plugin-import with eslint-plugin-import-x for flat config support
- Add globals package for environment globals
- Update eslint-plugin-bsky-internal with proper meta objects for ESLint v9
- Fix deprecated context.getScope() API usage
- Update bskyembed to use flat config
- Remove deprecated --ext flag from lint scripts
- Configure rules to maintain previous behavior while using new ESLint version

* Fix varsIgnorePattern to require character after underscore

Restore the original pattern `^_.+` instead of `^_` so that lingui's
`const { _ } = useLingui()` will still be flagged when unused.

* Update ESLint rule tests for flat config format

- Update RuleTester to use flat config languageOptions instead of
  eslintrc parser format
- Remove duplicate test case that ESLint v9 now detects
- Add Jest globals for test files

* update eslint package versions

* lint android a11y

* enable typechecked rules, switch them to warn

* fix yarn lock ci

* Fix CI failure

* Remove unused globals?

* Organize a bit, add quiet to main lint command

* Allow ternary

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Eric Bailey <git@esb.lol>

bskyembed/package.json

@@ -7,7 +7,7 @@
     "dev-snippet": "tsc --project tsconfig.snippet.json && serve -s dist -p 3000 -n",
     "build": "tsc && vite build",
     "build-snippet": "tsc --project tsconfig.snippet.json",
-    "lint": "eslint --cache --ext .js,.jsx,.ts,.tsx src",
+    "lint": "eslint --cache src",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
@@ -17,11 +17,13 @@
   "devDependencies": {
     "@preact/preset-vite": "^2.10.2",
     "@vitejs/plugin-legacy": "^7.0.0",
+    "@eslint/js": "^9.18.0",
     "autoprefixer": "^10.4.19",
-    "eslint": "^8.19.0",
-    "eslint-config-preact": "^1.3.0",
-    "eslint-plugin-simple-import-sort": "^12.0.0",
+    "eslint": "^9.18.0",
+    "eslint-plugin-simple-import-sort": "^12.1.1",
+    "globals": "^15.14.0",
     "postcss": "^8.4.38",
+    "typescript-eslint": "^8.20.0",
     "serve": "^14.2.5",
     "tailwindcss": "^3.4.3",
     "terser": "^5.43.1",