Support positional-only parameters (/)

[eallender]

Description:
Add support for Python 3.8+ positional-only parameters denoted by / in function signatures.

Example:

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

Implementation notes:

• Tree-sitter node kind is positional_separator
• Parameters before / should be marked as positional-only
• Update collect_args to track separator state
• Update docstring format to indicate positional-only status