Node.js worker thread API

[jasongin]

• Add NodeWorker class to wrap the Node.js worker thread API
  • At the moment there is a limitation in that it doesn't support JS/.NET interop in the worker thread. (The JS worker can still communicate with the .NET host via messages.) Direct interop in the worker will require some of the changes in the libnode embedding API that @vmoroz is working on now, specifically to allow the embedding host to provide modules.
• Add NodeProcess class to wrap a small subset of the Node.js process API, which is useful for workers (mostly for C# interop in workers which is not done yet).
• Add JSObjectDictionary adapter that supports adapting a JS object to an IDictionary with value conversions.
  • It's similar to the Map dictionary adapter except JS objects have slightly different semantics than maps.
  • This is used with the NodeProcess.Env to project the env JS object as IDictionary<string, string>.
• Fix a bug in NodeStream.ReadAsync(), and improve validation in the NodeStream constructor.

[vmoroz]