Add type hints to fluent.syntax & fluent.runtime

[eemeli]

Fixes #165

The work here is split into commits that may be easier to review separately. The internal hints for the runtime resolver and compiler are pretty weak, as the base implementation is making explicit use of the language being fundamentally untyped.

The bugs that are fixed are all various sorts of corner cases that the code didn't account for previously; the errors tend to follow similar cases in fluent.js.

While adding the types I used both mypy and pylance; the added CI tests only apply mypy --strict as the strict-mode pylance checks don't all pass (mostly due to using Unknown values in places that accept Any).

[parmenashp]

Are you planning on bumping to 0.19 after this?

[eemeli]

Yes.