Add support for providing document symbols

[dcermak]

This is a bit crude implementation of the document symbol capability leveraging the new AST.

Current drawbacks:

• everything is stuffed into the AST classes (I think this is reasonable, but it could become a mess if we add all the capabalities into the node classes)
• The selection range (range identifying the "main" symbol) is atm computed from the start of the node to start + len(identifier). This should be good enough, but maybe we should extract this information from the yaml parser?

[cbosdo]

👍