feat(developer): LexicalModelCompiler now implements KeymanCompiler

Relates to #9473.

Refactors the public API of LexicalModelCompiler to meet KeymanCompiler,
including asyncing a bunch of functions, and moving file write
responsibilities into the class itself. Removes top-level
`compileModel()` and `loadFromFilename()` functions, as you should now
use `LexicalModelCompiler` API.

Most of the test cases needed only minor patching, but introducing the
`async init()` function has caused them to all be async, despite the
model compiler itself having no async requirements. I think this is
fine, because it makes any future additional async initialization tasks
much less painful to implement.