Highlight npm run watch

[EdmondChuiHW]

Summary

There's a watch script in package.json that rebuilds automatically upon file changes.

Example usage in dev

• python3 -m http.server 8000 --directory out/Default/gen/front_end in the one terminal;
• npm run watch in another

It's not hot-reload, but it's an improvement from running npm run build on each file save.

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.