Add support for debugging servers in the VSCode Playground

[alcarney]

This adds back the ability for language servers to be debugged through the example VSCode extension.
Rather than make the user launch the client and server separately and using a TCP connection, the extension now uses the official Python extension to wrap the server in a debug adapter and automatically starts the debug session.

To debug the server set the pygls.server.debug option to true.

This will go towards #424 (though the docs will still need to be updated)

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly