Declare Command

The problem

Curent PyMOL approach to new plugin commands are outdated.

The proposal

Introduce a new system based on modern Python with type checking and implicit conversion.

What works right now?

On PyMOL open-source, but not on Incentive, there's a cmd.declare_command but isn't very much working for all possible cases.

Example of how it can be

@declare_command
def new_command(
    dirname: Path = '.',
    nullable_point: Optional[Tuple[int, int, int]] = None,
    my_var: int | float
    extended_calculation: bool = True,
    old_style: Any = "Support anything currently not supported"
) -> IgnoredType:
    """
    A cool docstring.
    """
    pass

I'm also propose to better support docstrings.