Bug: unclosed chokidar watcher on dev server shutdown

[wpj]

Using snowpack@3.0.0-rc.2, the shutdown function for stopping the Snowpack dev server isn't closing a persistent chokidar watcher, and as a result, the node process remains open even after calling shutdown. I've created a minimal repro here.

From what I can tell, this is the watcher that needs to be closed:

https://github.com/snowpackjs/snowpack/blob/6ce35ef870a9c27fba523357b364b370cae7f902/snowpack/src/commands/dev.ts#L1326