Add support for TypeScript
Fixes #5 (closed).
TypeScript is a superset of JavaScript, so the added lang simply extends the existing JS lang with TS specific stuff (adapted from how the C++ lang extends the C lang).
Shared MIME info doesn't define a MIME type for TypeScript, but there is a recently open issue for it: https://gitlab.freedesktop.org/xdg/shared-mime-info/issues/117. I adapted the MIME types from JavaScript into TypeScript MIME types, but I'm open to changes.