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.
Replace the use of the deprecated `Lock` from swift-tools-support-core
with `NSLock`. The benefit of the deprecated interface was the
`withLock` function which is easily replicated and very unlikely to
change. By doing so we avoid the warnings about `Lock` being deprecated
when building SourceKit-LSP.
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.
