Drop built-in LanguageClient, rely on the client provided by pygls

[alcarney]

This depends on a version of pygls that does not exist yet.

By using the LanguageClient provided by pygls we get to further simplify pytest-lsp's architecture.

• The server process is an asyncio.subprocess.Process and managed by the event loop (Closes Investigate child watchers #59)
• We no longer have to continuously probe the server process to see if it is still alive
• We no longer have to use a thread pool to asynchronously read from the server's stdout