Support for #if JAVASCRIPT

[Jand42]

Sometimes client-side code has to diverge from the server-side, and using conditional compilation symbol would be more flexible than current options.

Add a ws.config option UseJavaScriptSymbol to run the JS-side parsing with the JAVASCRIPT symbol defined. The new compiler option is needed so that if this feature is not needed (by default), FSC does not need to parse and typecheck the project twice.

This also helps internally to eliminate the currently separate dummy project for WebSharper.Main.Proxies.