Add the ability to look-at in model-space.

This is a much simpler attempt to solve the same problem as #76060, but without breaking any compatibility.

* Adds a description of what model space is in the Vector3 enums (MODEL_* constants). This has the proper axes laid out for imported 3D assets.
* Adds the option to `look_at` using model_space, which uses Vector3.MODEL_FRONT as forward vector.

The attempt of this PR is to still break the assumption that there is a single direction of forward (which is not the case in Godot)
and make it easier to understand where 3D models are facing, as well as orienting them via look_at.

doc/classes/Transform3D.xml

@@ -93,6 +93,7 @@
 			<return type="Transform3D" />
 			<param index="0" name="target" type="Vector3" />
 			<param index="1" name="up" type="Vector3" default="Vector3(0, 1, 0)" />
+			<param index="2" name="use_model_front" type="bool" default="false" />
 			<description>
 				Returns a copy of the transform rotated such that the forward axis (-Z) points towards the [param target] position.
 				The up axis (+Y) points as close to the [param up] vector as possible while staying perpendicular to the forward axis. The resulting transform is orthonormalized. The existing rotation, scale, and skew information from the original transform is discarded. The [param target] and [param up] vectors cannot be zero, cannot be parallel to each other, and are defined in global/parent space.