feat(developer): KeyboardInfoCompiler now implements KeymanCompiler

Relates to #9473.

Refactors the public API of KeyboardInfoCompiler to meet KeymanCompiler,
including moving file write responsibilities into the class itself.

`sources` has become a property of `options`, which is perhaps a slight
violation of the intent of the `init()` function being a one-time call,
as it means we'll need to instantiate an new class for each file we
compile. Given the use-case for the keyboard_info compiler is almost
exclusively internal, I think this is acceptable. Tell me if you think
otherwise!