[WIP] Fable JS support

[ncave]

• Compiling Fable with Fable works
• Babel AST JS output works
• Demo JS page works
• Next step: To be determined by community, open for suggestions.

Disclaimer: This is a rough POC, corners have been cut to get to MVP faster.
That said, it works (but then we'll have to go back and add some corners).

Dependency: FCS for Fable
Build instructions: TBD

[ncave]

@alfonsogarciacaro I think this is ready for merging, please let me know what you think.
It does not affect normal Fable as all code is behind #ifdefs, and all tests are passing.
Also, we can start working on a Fable Live JS page, it will be a nice showcase for Fable.

[alfonsogarciacaro]

Awesome work @ncave, thanks a lot for this! 👏 👏 👏

I've created a new branch to start working in the release of Fable 1.0 (codename narumi) to unify the .NET/Mono and Netcore versions and include the integration with dotnet SDK #584. I think it may be a good idea to make the merge directly in that branch. If you agree, could you please change the target of the PR?

[ncave]

@alfonsogarciacaro Target changed.