[feature] serve to allow pushing custom events to /esbuild SSE

Serve is almost pefect for me.

I have a simple function that uses glob to sync static assets into build folder.

I can run both server and my watcher so esbuild handles bundling, and my watcher syncs static files.

After I copy changed static asset, I could call some kind of `ctx.notify()` that would be pushed via `/esbuild` SSE, I would not need `live-server` anymore.

Start by tracing the serve entry point and the /esbuild SSE endpoint mentioned in the issue. Determine how a custom notification event should be represented and delivered; done means an external watcher can trigger it and connected SSE clients receive it without live-server.

Written by the indexing model from the issue text.