Built-in TypeScript types

[bergwerf]

Having TypeScript types would be super useful for TypeScript users. Currently the types as https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/opentype.js are outdated (missing at least font variations). For us types are the starting point to figure out what a library can do. If they are included in the library, that saves a second dependency and ensures they are always synchronized with the latest version.