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.
Naming types in sourcekitd_functions.h `sourcekit_api_` instead of `sourcekitd_` indicates that these are types to be used with dynamically loaded sourcekitd libraries. It avoids confusion if sourcekitd is also linked, which adds the `sourcekitd_` symbols.
Adding nullability annotations to it is also just nice.
And some improved formatting never hurts.
