Rename GlListboxItem to GlListboxOption

The following discussion from gitlab-org/gitlab-ui!2988 (merged) should be addressed:

• @markrian started a discussion:

  I think it's out of scope of this MR, but I think this should really be called GlListboxOption, since they are role="option".

The above shouldn't be breaking change, since GlListboxItem isn't exported by the package.

We might also want to consider renaming anything called item to option, but these would all be breaking changes:

• The items prop -> options. But, maybe items is a reasonable name, since it can receive options or groups?
• The list-item slot -> option, and passing { option } rather than { item }