Rename PathFollow's offsets to progress & progress_ratio

[Mickeon]

Partially closes godotengine/godot-proposals#5104

As brought up in #54161 (comment).

Properties containing "offset" are very vaguely utilised across the language. Often that's fine, but for both PathFollow2D and PathFollow3D it's particularly egregious. There's offset, unit_offset, h_offset and v_offset. The latter two refer to the visual offset, and unlike other classes (such as Sprite2D), these cannot be unified as a single Vector2 value, because the singular offset property already exists, referring to the offset along the curve. This PR attempts to fix one part of that.

For both PathFollow2D and PathFollow3D:
offset -> progress
unit_offset -> progress_ratio

I find the name "progress" to be quite suitable and a much more specific term. Think of "The progress along the track, measured in units of distance".
Meanwhile, "ratio" fits a lot more than "unit" to to imply the range between 0.0 and 1.0. The new name also allows it to be grouped better along with "progress" This name would also bring more consistency with this other PR: #64665

Applies for both PathFollow2D and PathFollow3D. Updates documentation and project converter, as well.

[Zireael07]

First I've heard of progress being used in the update loop aka _process or _physics_process.

[akien-mga]

Approved in PR review meeting. We bikeshedded for a while as we weren't fully convinced by the new names, but it's definitely better than the current confusion with offset, h_offset and v_offset.

[akien-mga]

Thanks!