feat(repair): add lerna repair command

[JamesHenry]

Description

Adds a new command: lerna repair. This command will run any available migrations which have been designed to bring a lerna workspace up to date with the latest and greatest.

This PR does not ship with a real implementation of such a migration, so instead a noop example migration is provided to concretely illustrate how a migration is both written and unit tested within the lerna package.

NOTE: the implementation for the repair command is intentionally inlined into the lerna package. Because of the way migrations are orchestrated, having repair be its own package would result in a cyclical dependency between it and the core package. Plus, in general, we are not looking to continue with the pattern of having an ever-increasing number of tiny published packages.

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]

Looks good. Looking forward to this command!