AST/ASR views: allow to click on the node to show position in the code

[certik]

In the AST/ASR views, we should allow to click on the AST/ASR nodes, and when you do, it will highlight the given span in the code on the left. Also allow moving the the previous/next node using left/right arrows.

And going the other way: move the cursor in the code window and automatically highlight the AST/ASR node that corresponds to it (the nodes are typically nested, so more than one can correspond to a given cursor position, so we should highlight the one with the shortest span).