workflows/labels: add "6.topic: tree-sitter" label
Add automatic labeling for tree-sitter related changes including:
- Tree-sitter core parsing tools
- Python tree-sitter modules and bindings
- Vim/Neovim tree-sitter plugins
- Emacs tree-sitter packages
- Tree-sitter documentation
(cherry picked from commit e8518435e8a38113be58d4f74d622b61a9ee5830)