feat(demo, docs): introduce threading test demos for browser and fallback, enhance threading documentation

- Added `test-browser-worker.html` to demonstrate threading with Web Workers in browser environments.
- Added `test-fallback.html` to verify fallback functionality when threading is unavailable.
- Created `THREADING.md` to document unified threading implementation, including Node.js Worker Threads, Web Workers, and fallback mechanisms.
- Updated Node.js version requirement in `README.md` and `package.json` to `>=24.0.0` for compatibility with improved Worker Threads API.
- Enhanced `workerUtils.ts` to implement threading with a worker pool for Node.js and added Web Worker execution logic for browsers.
- Enabled environment-aware threading availability in `environment.ts`.
- Updated `rollup.config.js` to include CLI configurations for streamlined builds.
- Adjusted `.gitignore` to exclude test artifacts and package files.

These changes provide comprehensive testing and documentation of threading functionality, improve cross-environment compatibility, and enhance developer workflows.

src/index.ts

@@ -33,12 +33,22 @@ import {
   defaultEmbeddingFunction
 } from './utils/embedding.js'
 
+// Export worker utilities
+import {
+  executeInThread,
+  cleanupWorkerPools
+} from './utils/workerUtils.js'
+
 export {
   UniversalSentenceEncoder,
   createEmbeddingFunction,
   createTensorFlowEmbeddingFunction,
   createThreadedEmbeddingFunction,
-  defaultEmbeddingFunction
+  defaultEmbeddingFunction,
+
+  // Worker utilities
+  executeInThread,
+  cleanupWorkerPools
 }
 
 // Export storage adapters