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/eslint.config.mjs

@@ -0,0 +1,52 @@
+// @ts-check
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+import simpleImportSort from 'eslint-plugin-simple-import-sort'
+import globals from 'globals'
+
+export default tseslint.config(
+  // Global ignores
+  {
+    ignores: ['dist/**', 'node_modules/**'],
+  },
+
+  // Base JS recommended rules
+  js.configs.recommended,
+
+  // TypeScript rules with type checking
+  ...tseslint.configs.recommendedTypeChecked,
+
+  // Main configuration
+  {
+    files: ['**/*.{js,jsx,ts,tsx}'],
+    plugins: {
+      'simple-import-sort': simpleImportSort,
+    },
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      globals: {
+        ...globals.browser,
+      },
+      parserOptions: {
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    rules: {
+      'simple-import-sort/imports': 'warn',
+      'simple-import-sort/exports': 'warn',
+      'no-else-return': 'off',
+      '@typescript-eslint/no-require-imports': 'off',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          argsIgnorePattern: '^_',
+          varsIgnorePattern: '^_.+',
+          caughtErrors: 'none',
+          ignoreRestSiblings: true,
+        },
+      ],
+    },
+  },
+)