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

eslint/use-prefixed-imports.js

@@ -13,7 +13,11 @@ const BANNED_IMPORT_PREFIXES = [
 module.exports = {
   meta: {
     type: 'suggestion',
+    docs: {
+      description: 'Enforce using prefixed imports for internal paths',
+    },
     fixable: 'code',
+    schema: [],
   },
   create(context) {
     return {