Convert OpenFeature provider to TypeScript with npm workspaces setup

- Move openfeature-server-provider to packages/ as an npm workspace
- Convert all source and tests from JavaScript to TypeScript
- Add README adapted from the Java OpenFeature provider
- Bump main SDK version to 0.21.0 for shutdown method support
- Add shutdown() and areFlagsReady() to flag provider type declarations
- Add workspace scripts (test:all, build:workspaces) to root package.json
- Exclude packages/ from main SDK npm tarball via .npmignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: msiebert

.npmignore

@@ -1,2 +1,3 @@
 test/
 .github/
+packages/

lib/flags/local_flags.d.ts

@@ -77,4 +77,14 @@ export default class LocalFeatureFlagsProvider {
    * @param context - Evaluation context (must include distinct_id)
    */
   getAllVariants(context: FlagContext): { [key: string]: SelectedVariant };
+
+  /**
+   * Stop polling and clean up resources
+   */
+  shutdown(): void;
+
+  /**
+   * Returns a promise that resolves when flag definitions have been fetched
+   */
+  areFlagsReady(): Promise<void>;
 }

lib/flags/remote_flags.d.ts

@@ -71,4 +71,9 @@ export default class RemoteFeatureFlagsProvider {
   getAllVariants(
     context: FlagContext,
   ): Promise<{ [key: string]: SelectedVariant } | null>;
+
+  /**
+   * Clean up resources (no-op for remote provider)
+   */
+  shutdown(): void;
 }