Add `.swift-format` to the repo and format the repo with `swift-format`.
This commit does not add any automation to enforce formatting of sourcekit-lsp in CI. The goal of this commit is to get the majority of source changes out of the way so that the diff of actually enforcing formatting will have fewer changes or conflicts.
The LSP spec requires us to omit keys for nil values rather than using
the JSON `null` constant in most places. This change to CustomCodable
allows us to do it automatically using CustomCodable, removing one of
the limitations of the property wrapper.
The idea for the adding overloads of `encode` and `decode` came from
https://forums.swift.org/t/pre-pitch-codable-customization-using-propertywrappers/30244/
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
