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
The style I started with (putting each conformance on its own extension)
turned out to be pointlessly verbose in practice. It also separated the
protocol from the "implentation" in the sense that the property
declarations are what determine how the conformance works.
