Implement textDocument/foldingRange handler

[retailcoder]

The client-sided proof-of-concept code produced a ParseTreeListener that should be relatively easy to tweak to output LSP-compliant FoldingRange items (only for the active document) instead of AvalonEdit model items - on the client side the LSP items should be converted into the corresponding AvalonEdit objects, and rendered in the active document.

• Modify the listener to use the LSP-compliant model types
• Attach the listener to the parser that processes the active document
• Return the folding ranges in the response

As with all LSP requests, the server operates with the version of the code that is currently synchronized at the moment of the request.