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.
Explicitly import interfaces from TSCBasic which now allows us to
identify all the swift-tools-support-core interfaces which are in
use in SourceKit-LSP.
