feat(add): New command to add packages without reifying entire tree

[darcyclarke]

Summary

npm add <pkg> should be a separate command from install (right now it's aliased) & be the only way you "add" a package to your project. npm install would then no longer accept arguments (as of v9). This clear delineation will help ensure we can focus on the correct functionality for each of these use cases individually (while also reintroducing v6 behavior).

Exit Criteria

• npm add function is decoupled from install
• install no longer supports positional arguments
• npm add requires declaration of a <pkg>
• tests are written to cover the functionality & use cases
• documentation is updated to cover the functionality & use cases