Improve undo redo API and docs

[Goldenlion5648]

The API was a bit inconsistent and unclear. This attempts to fix some of that with examples and aliases for existing API methods, and exposing discard_redo()

Fixes godotengine/godot-proposals#14539

The examples can be updated to use bind once #107756 gets merged

Maybe in a followup PR or this one depending on what people think:

• get_action_name could be get_action_name_at_index
• Add a built in way to print the UndoRedo object (just the names of the states, and maybe function names?)
• The editor inspector could show the state names, currently if the godot program hits a breakpoint in the editor, it is unknown what the state of the undo redo history was

[AThousandShips]

I don't think the new method aliases is a good idea in addition to these changes, but my request for changes is about the documentation errors

[Goldenlion5648]

If we wanted to totally replace the existing methods, is there a preferred way to deprecate the old ones?

[AThousandShips]

Yes to deprecate them, but I don't think there's any real strong reason to do so here, it creates clutter and confusion I'd say, so there would need to be some concrete demand or need for the change, is there a lot of confusion over these names?

[MewPurPur]

Sorry to shoehorn, but I have a proposal to turn UndoRedo into a RefCounted class, to avoid the need to free it manually or otherwise get memory leaks: godotengine/godot-proposals#11868

I'm not saying this PR has to be a full rework of UndoRedo, I wouldn't have mentioned it if the PR was just adding a new method without also doing aesthetic renames and deprecating stuff. But since it does, it seems like a good opportunity to fix the mistakes from the initial implementation in one swoop.