Leverages the new "views" API in VS Code to show a tree view in the file
explorer sidebar. Clicking a tree node navigates the editor to the line
containing the source element/attribute. The tree can be navigates via
keyboard by using the UP/DOWN arrow keys to choose nodes and the
LEFT/RIGHT arrow keys to collapse/expand nodes.
Addresses the following issues:
#62#66#71#72#73#83#102
