Add HeightMapShape3D functions to get min / max height

[smix8]

Adds HeightMapShape3D functions to get min / max height.

Not sure why those were never exposed while already tracked on the shape. Knowing what is the min and max height in the heights array is very useful information to avoid looping over the entire large array manually.

Also there are no longer pool arrays in Godot 4, they are now packed arrays, but I don't see much value in mentioning this in the text when the function makes it obvious.

[Mickeon]

Docs are nice. I can somewhat speak on implementation? These values already existed internally, this PR just exposes them.

[kleonc]

Exposing these makes sense to me.
(+I think I've already seen these values being calculated manually in some user code; not sure if I'm not making this up though 😆)

[akien-mga]

Thanks!