[3] refactor: improve & simplify tree-sitter context-tracking

[juleswritescode]

• introduced a new scope_tracker that makes context tracking easier and will allow us to discard context when we enter subqueries.
• ctx.node_under_cursor does not need to be an Option; we should always have a tree.root_node
• deleted a couple of obsolete helper methods on the TreesitterContext

Next up, I'll make sure we open new scopes for CTEs, subqueries etc.
Also, I think the whole WrappingClause and WrappingNodes are unnecessary indirections – we can just as easily check the tree-sitter node kind &strs.

[psteinroe]

seems like the compiler book starts to show its influence :D