Refactor embedding API and code to separate assembly

[jasongin]

Proposal:

• Create a new assembly: Microsoft.JavaScript.NodeApi.Embedding and nuget package
• This assembly / package will have a dependency on the LibNode package for the native embedding runtime
• Move the public embedding types (NodeEmbedding*) into that assembly / namespace.

Probably there will still need to be some embedding-related code in the core assembly, particularly JSRuntime. I think that's OK.