Context menu for tree nodes

[Rekkonnect]

Bring up a context menu when right-clicking a tree node. Include the standard actions that are already implemented and more:

• Delete node from source (mutates the source such that the syntax node corresponding to the selected node is deleted)
• Destroy loaded descendant nodes (clears out the descendants in the loaded analysis tree, which can be good for performance)
• Generate discovery expression (generates a C# expression for syntax and symbol nodes that discovers the selected node from the compilation unit or global namespace roots respectively)