When receiving a `CancellationNotification`, we cancel the task that handles the request with that ID.
This will cause `cancel_notification` to be sent to sourcekitd or a `CancellationNotification` to be sent to `clangd`, which ultimately cancels the request.
rdar://117492860
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.
