`multi` should be parsed as terminal

[darosior]

When parsing a multi from Script we treat it as a non-terminal fragment. It's a terminal so it should be treated as such.

In this function: https://github.com/darosior/python-bip380/blob/7b79c4719d3d311dec9c7bc5e6d63c79608a3b42/bip380/miniscript/parsing.py#L255-L260

https://github.com/darosior/python-bip380/blob/7b79c4719d3d311dec9c7bc5e6d63c79608a3b42/bip380/miniscript/parsing.py#L289-L319