Support keyword-only parameters (*)

[eallender]

Description:
Add support for keyword-only parameters denoted by * in function signatures.

Example:

def foo(a, b, *, c, d):
    """
    Args:
        a (type): Description.
        b (type): Description.
        c (type): Description, keyword-only.
        d (type): Description, keyword-only.
    """

Implementation notes:

• Tree-sitter node kind is keyword_separator
• Parameters after * should be marked as keyword-only
• Distinguish from *args (which is list_splat_pattern)
• Update collect_args to track separator state
• Update docstring format to indicate keyword-only status