Reorganize and rename JavaScript and TypeScript variants

[martin-henz]

Currently we have three languages at toplevel: JavaScript, Scheme, Python. Under JavaScript, we have the various Source and Source Typed languages.

I propose that we rename Source to JavaScript and Source Typed to TypeScript. TypeScript should be provided at the toplevel, so we have JavaScript, TypeScript, Scheme, and Python. Under JavaScript, we will have JavaScript §1, etc and all the variants, and under TypeScript, we will have TypeScript §1, etc and all the variants.

This way, "Source Academy" becomes visibly polyglot, and our audience will not associate Source Academy with any particular language that is supported by the system.