Rename some properties of PathFollower3D and PathFollower2D

[xiongyaohua]

Describe the project you are working on

A road traffic game, where roads are described by 2D or 3D curves. Curves are constantly sampled in order to convert the position of cars from linear coordinate to descartes coordinate.

Describe the problem or limitation you are having in your project

In the process of moving rotation interpolation code from PathFollower to Curve(see PR #63956, PR #64043), I feel some naming of properties are not clear.

1. On PathFollower3D:

   • h_offset: the doc says "The node's offset along the curve". But it is incorrect, it actually represents the offset along the "Sideway"(right hand) direction in the code.
   • v_offset: the doc says "The node's offset perpendicular to the curve". The concepts of "vertical" or "perpendicular" make sense for 2D curve but are ambiguous for 3D curve. It actually represents the offset along the "Up" direction in the code.
   • for completeness, there should be an offset along the tangent direction(or "Forward" direction in the code), but it is missing.

2. On both PathFollower2D/3D: the offset property indicate the distance moving from the beginning along the curve, which have different quality as h_offset or v_offset, as they move along straight vector. Use the same word "offset" could create confusion for new user.

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

for PathFollower3D
combine v_offset ,h_offset, f_offset into a Vector3 offset

for PathFollower2D
combine v_offset, h_offset into a vector3 offset,

for both PathFollower3D and PathFollower2D
- change offset to progress,
- change unit_offset to progress_ratio

The new names feel more consistent and clear.

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

I can make this change in a separate PR.

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

no

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

they are part of the engine API