Add Stair-Stepping upwards for CharacterBody3D

[Ruahsu]

Describe the project you are working on

A simple Quake-like capture the flag game.

Describe the problem or limitation you are having in your project

Stair-stepping upwards, a feature already included in other engines, like Unity and Unreal, is absent in Redot, despite stair-stepping downwards already existing currently. (floor_snap_length)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Stair-stepping instantly snaps you up to a ledge that is under a defined maximum height if you move into it. While there are solutions for this made in GDScript, an actual, true hardcode implementation has yet to be added to Godot, despite the already existing issue in godot-proposals laying dormant since 2021. godotengine#2751

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

An added toggle to the Floor category in CharacterBody3D, allowing the ability to define a maximum step-up height, as well as whether or not the body is allowed to step-up stairs midair, preserve their upwards momentum while doing so, along with a function or signal that returns true when a step-up is performed on that physics tick.

If this enhancement will not be used often, can it be worked around with a few lines of script?

It is a simple toggle.

Is there a reason why this should be core and not an add-on in the asset library?

Unity, and Unreal already have this implemented, and Redot currently only has half of the feature. Some developers might not want to make their own stair-stepping solution, use workarounds like disguising stairs as ramps, and already existing solutions all have their issues and differences in implementation. A single, proper, easily toggle-able option already in-editor would solve these issues.