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.
For now, we only have one concrete implementation in sourcekit-lsp
itself, but it is useful for libraries to be able to wrap their own
sourcekitd implementation for example if they have an existing external
use of sourcekitd in C code.
