Support pnpm

[ExE-Boss]

pnpm is the third largest node package manager, which uses symlinks, hardlinks and isolated trees, which prevents packages from using packages which they haven’t declared in their dependencies and it uses shrinkwrap.yaml as its lockfile.

[janl]

We’ll look into this eventually. In the meantime, we’d be happy to accept PRs for this.

[ExE-Boss]

pnpm has a published specification for this at: https://github.com/pnpm/spec/tree/master/shrinkwrap

[qm3ster]

Updated link, since in 3.0 it was renamed to lockfile and is now stored as pnpm-lock.yaml
https://github.com/pnpm/spec/tree/master/lockfile

[qm3ster]

@ExE-Boss I tried in #229