Deprecate, then remove the ItemModifier and ItemDecorator syntax extension traits

[nrc]

In favour of the more flexible MultiItemModifer and (in the future, hopefully, see #21154) MultiItemDecorator. We should add some convenience methods to make these as ergonomic to use in the item case as the current interfaces.

[nham]

Triage bump: Seems these were deprecated about a month ago.

[steveklabnik]

Triage: this appears to be done. Closing!