I'm not aware of an in-built way to slice a spritesheet into individual files. That said, there are any number of spritesheet slicer tools available (both commercial and free) in the wild - unrelated to Godot. One of them might be the easiest solution.
Otherwise, it probably wouldn't be too difficult to write a
tool script for Godot to do the heavy lifting.
Also, you could also check Godot's Asset Library for something of interest. There are at least a few things listed there that could be helpful (?).
There's also this video...