[rush] rush add --peer

[elliot-nelson]

Summary

Add --peer flag for rush add, for adding peer dependencies.

Fixes #1456.

Details

• Extend existing functionality to also include peer dependencies.

How it was tested

In local monorepo:

• Confirmed 'rush add --peer' can add a peer dependency
• Confirmed specifying '--dev --peer' is not allowed
• Confirmed 'rush add --peer' on a dep used in other packages correctly selects existing version
• Confirmed 'rush remove' correctly removes a peer dep (and deletes peerDependencies section if appropriate)

Impacted documentation

• https://github.com/microsoft/rushstack-websites/blob/main/websites/rushjs.io/docs/pages/commands/rush_add.md