support a --legacy-peer-deps option

[rubys]

See Error while deploying Node.js app for details.

Summary: a package can indicate peer dependencies, and often this is wrong.

npm will, by default, respect these dependencies unless --legacy-peer-deps is passed as an option. When needed, it has to be on both the install and prune options.

yarn v1 (the one pretty much everybody uses) will ignore these dependencies. Yarn v2 and v3 respects them, unless --legacy-peer-deps is passed as an option.

pnpm ignores these dependencies unless --strict-peer-dependencies is passed as an option.

This seems to be a problem that very few people hit, but enough of them do to merit a flag.