We don’t have any guarantees which thread these blocks will be called on by sourcekitd, so we shouldn’t make any assumptions about it in Swift. We should thus mark them as Sendable.
This adds a sourcekitd plugin that drives the code completion requests. It also includes a `CompletionScoring` module that’s used to rank code completion results based on their contextual match, allowing us to show more relevant code completion results at the top.
