Support rule groups

[lvignoli]

Currently, rule groups are not supported and even produce a parse error, — breaking any highlighting based on it.

Example:

[group('python')]
fmt:
    ruff format || true

This is the output from Zed's AST using jackTabsCode/zed-just which leverages tree-sitter-just.

(An example from the tree-sitter playground would have been more informative, but I cannot compile tree-sitter-just locally, I'll try again but platform specific issue it seems.)