Remove `settings(for:)` from the BuildSystem API in favor of the change callback
- With the `settings(for:)` method removed, we instead block on a
callback from the `BuildSystemManager` which itself handles
the primary build system callback in conjunction with the fallback
system
- We queue up notifications / requests that reference documents
until we receive the callback.
- Remove language from FileBuildSettings as it is no longer needed,
Swift-specific working dir fixes are now in SwiftCompileCommand
- registerForChangeNotifications should inform its delegate
asynchronously instead of immediately
Request types should always have the suffix Request and notifications
should end with Notification.
Also moved all request and notification types into separate folders to
reduce the number of files in the LanguageServerProtocol folder.
