allow syntax::ext::base::MultiItemModifier to return multiple items

[llogiq]

As of yet, MultiItemModifiers can only return a single item, which can lead to problems with expanding macros or preprending items, e.g. extern crate foo; use foo::bar to avoid requiring users to manually import items.

A simple solution would be to change Annotatable to work on SmallVecs of Items/TraitItems/ImplItems. However, this would break all procedural macros using this interface.

[jseyfried]

@llogiq c.f. #34253