feat(users/Profpatsch): moving around via the tree-sitter parse tree

Has a little setup to get the cursor position and map it onto a tree
sitter node. The current node is saved in a cursor variable, and a
highlight overlay marks the range of the current node in the buffer.

Change-Id: I0af56115f928732e993fbefe978a246ca7c757ee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2258
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

third_party/default.nix

@@ -149,6 +149,7 @@ let
       texlive
       thttpd
       tree
+      tree-sitter
       unzip
       which
       writeShellScript