Move chokidar to peerDependencies and make it optional via peerDependenciesMeta

[rdmurphy]

Summary

Proposed change:

This pull request implements the discussed solution to #1227 and moves chokidar from optionalDependencies (which never did what it aspired to do) to peerDependencies, and uses the recently implemented peerDependenciesMeta field to mark it as optional. This means for npm users using a version that supports peerDependenciesMeta, chokidar will have to be separately installed. This is in particularly relevant to the newest version of npm (v7), which installs peerDependencies by default unless the package is marked as optional.

Closes #1227.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[fdintino]

Thank you for this!