Apart from removing unnecessary imports, the following changes were
performed:
- Move CancellationToken from SKSupport to LanguageServerProtocol
- Move CustomCodable to its own module
- CustomCodable belongs in LanguageServerProtocol but moving it there
causes compilation to fail. As a workaround, move it to its own
module
This lets us use the `Range<Position>` without manually converting back
and forth in the majority of cases. See the note on HoverResponse for
the behaviour with Optional that makes us have to provide a custom
Codable implementation in two places that used to be synthesized.
