Add build-packages:watch script

[nbloomf]

Changes proposed in this Pull Request

• Adds build-packages:watch script, which uses chokidar-cli to watch and rebuild source files in packages/.
• Includes build-packages:watch with main watcher scripts during poststart.

Testing instructions

Run npm start, then make changes somewhere in packages/

[matticbot]

Test live: https://calypso.live/?branch=add/build-packages-watch

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirbrillig]

Works as described. The only downside I see is that it rebuilds all of the packages if any one changes, but I'm not sure if there's an easy way around that. Maybe if we added a separate watch script for each package, but I don't know if the downside is worth that effort. 🤔 Still, I think it's better than what we have now.

[nbloomf]

it rebuilds all of the packages if any one changes

I agree this isn't ideal -- for one thing as TypeScript usage grows the cost of unnecessary tsc runs will eventually hurt. A more sophisticated script could probably handle this though; chokidar puts a stream of changed filenames on stdout, from which we could extract package names and call more specific build scripts. But as you say it's not obvious whether the added complexity is worth it right now.

[blowery]

Agreed, let's keep it fairly simple for now. Thanks for putting this together!

[blowery]

down the road, another option would be to add a watch or dev target to every package under packages and have that use the "best" method for each package. tsc already has a --watch option that we could use for typescript based packages.