Revisit outputting TypeScript declarations

[Jand42]

TypeScript declarations for WebSharper 7 need some more work, these are some things to fix:

• Make interface methods with default inline implementations optional in .d.ts. For example IEnumerable.GetEnumerator is proxied as such that it translates to a call to a Get0 method which checks first if object has GetEnumerator0, if not it uses GetEnumerator. This allows simplifying usual scenario where the two methods would return the same anyways, while simplifying runtime JS represenatation by having only GetEnumerator(). It could be argued that the two could be merged completely.

* Resolve import names to not collide with interface definition name, add generics. For example for generic `IEnumerable`:

* unions: if type translates to a plain object, with potential null/constant cases, export the full type shape as default, id lookup issues

* unions: if type translates to a class, create a `_T` type alias that is an intersection of the class type and the union cases: