Implement LSP command in CLI executable

[MaxDesiatov]

This requires #27 to be fully implemented.

CLI invocation of the form typology lsp should start an LSP server listening on some default port. This server should be able to typecheck isolated files and provide name resolution for declarations defined in those files. Resolving declarations across files and modules would be implemented as a part of a separate issue.

We can reuse the existing LanguageServerProtocol module in sourcekit-lsp. I'm linking to a fork as we will still need to make that module importable in Typology by defining it as a product in our fork.

To test the server, it makes sense to build a simple VS Code extension as described in Language Server Extension Guide. In the first implementation, we can start with supporting only DidSaveTextDocument notification to type check the opened file when it's saved and emit diagnostics for discovered errors.