Add 'gap' modifier from Uiua

[JoaoFelipe3]

I added this so we can have an easier way to pop values off.

[kaikalii]

I wonder if dip should be ; and , should be gap? Or : and .. Or some combination with an appropriate glyph for identity.
That way a chain like dip gap gap dip id looks like ;,,;: or :..:;.

[kaikalii]

Though now that I think of it, planet notation requires modifiers to run the opposite direction from functions.