Remove JSNativeApi.cs

[vmoroz]

This PR is a part of the code refactoring that aims to simplify the code and to improve public API.
In this PR we remove the JSNativeApi.cs file that mostly had extensions for the JSValue struct.
We still have JSNativeApi class in the JSError.cs file that we can remove in future PR.
The key parts of this PR:

• JSVativeApi.cs code is moved inside of the JSValue struct.
• Code that access JSRuntime functions in JSValue is changed to minimize required verifications. New private functions
  GetRuntime and GetCurrentRuntime return napi_env and napi_value in one call.
• Code that was dependent on the JSNativeApi is changed to use the new JSValue functions.
• JSMarshaller changes reflect the function changes from JSNativeApi static methods to JSValue instance methods.
• Some comment and spelling fixes.

In the next PRs we should improve the JSValue API and API for other classes and structs.
E.g. we can move some methods to JS type specific structs.

[jasongin]

Overall this is a great improvement to the public API surface.

I just have some concerns about keeping conversion APIs for JSBigInt consistent with other types.