You should already be able to multi-select your results (or items from a library screen). This works by manually selecting the individual items – or selecting the 1st, scroll to the last item and select that holding the shift key (⇧).
That being said… there’s an existing feature suggestion to add a “Select All” button similarly to the “Deselect All” option already available in the multi-select action bar. Unless I’m very much mistaken and you’re looking for something very different, I recommend you comment/vote in that thread in order to avoid distracting/dispersing votes (closing this thread as a duplicate).
