feat(watch): Add lerna watch command

[fahslaj]

Adds a command that supports watching for changes within Lerna packages.

Description

lerna watch can be used to run arbitrary commands when any of a package's files change on disk. See /core/lerna/commands/watch/README.md for examples.

Motivation and Context

This solves a common problem for package-based monorepos, which is to trigger rebuilds of packages when their files change.

How Has This Been Tested?

This has been tested manually as well as with dedicated e2e tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (change that has absolutely no effect on users)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[fahslaj]

@JamesHenry The issue was specific to Nx workspaces with a npmScope defined in nx.json. The watch command has been updated to properly mimic this behavior when nx.json has this property. An e2e test was also added to cover this case.

[JamesHenry]

🎉