Add npm dependencies?

Continuing discussion from https://github.com/fable-compiler/Fable/issues/1649#issuecomment-440917740

There are currently two issues for Fable users and authors. Would it be possible to fix both by adding npm dependencies to Paket?

• Fable users need to handle two package managers: Paket/Nuget and npm
• Sometimes Fable lib authors need npm dependencies and they have no way to specify that, so users need to check the docs and remember what they should install.

Note I'm not talking about Fable.React because it's loosely coupled to React (can be used with preact, or in a netcore app), but about other libs where the npm dependency is more transparent to the user (Fulma, Zaid's bindings...).

What would Paket do with npm dependencies?

• I assume Paket won't take care of the actual installation, just adding the packages to package.json (translating the semver syntax used by Paket to npm's one).
• Should Paket run npm install automatically after restore/install if it detects npm dependencies?

Start with the linked discussion in issue 1649 and this issue's questions about Paket, package.json, npm install, and semver translation. Define the intended behavior for declaring npm dependencies and whether restore/install should invoke npm; done means those behaviors and scope are agreed and documented for implementation.