fix: remove default item_width

[medanisjbara]

I'm not sure if you'd be interested in this change.

I have noticed that non-selected items seem to be showing in a separate line from the prefix.

This only seems to be the case for unselected items.

At first, this sounded like a terminal specific problem. But i have tried a bunch of terminal emulators (mainly foot and xfce4-terminal) and the problem seems to persist.

Note that the problem does not seem to depend on the value of item_width (I have tried values as big as 5000 and the problem still persisted).

I believe making item_width an option that is only passed to gum when set is a good solution for this problem. I am not sure if the problem used to exist with gum or not, and I think sending them a PR is a better fix than making a PR here (since it'd fix the problem at it's root). But I figured (since I'm already using this) that you might be interested in merging this until gum is fixed.