When saving an item, some logic is done to determine whether one can
save it. In the special case where the intent is to `SaveAs`, it was
previously allowed to proceed as long as the buffer was a singleton
(presumably since it only makes sense to provide a save path for a
single file). However, we need to _also_ check that this item can be
"saved as" at all.
For this, we resurrect the `ItemHandle`/`Item` trait method
`can_save_as`. We have given it the default implementation of returning
`false`, and then overridden this in the implementation for
`TerminalView`.
Closes#25023
Release Notes:
- Fixed crash when trying to save terminal buffer
---------
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
