Implement language server for Starlark

First off, I'm really excited to see the Bazel team creating a repository for vscode support and I would love to contribute to it!

Secondly, Is this Repository going to be developing a [Language Server Protocol](https://langserver.org/) implementation? Seems like it could be a great opportunity to develop a resource that could be reused in other editors as well.

The issue names no files, tests, or entry points. Start by reviewing the repository's existing VS Code support and the requested Language Server Protocol scope, then clarify whether a reusable Starlark language server belongs here. Done would require an agreed design and implementation scope before coding begins.

Written by the indexing model from the issue text.