Delete sourcekit-lsp's `Sources/SwiftExtensions/ThreadSafeBox.swift`
and the now-unused `NSLock+WithLock.swift`.
Use the `Mutex`-backed `ThreadSafeBox` from swift-tools-protocols'
Adjust call sites for the new API: e.g.
`foo.value.mutate()` with `foo.withLock { $0.mutate() }` because it
doesn't provide `_modify` accessor.