Add a button to reverse a Gradient resource in the editor

[Xrayez]

Describe the project you are working on:
Goost - Godot Engine Extension.

See godotengine/godot#42855 and #1677.

Describe the problem or limitation you are having in your project:
See godotengine/godot#42855 (comment).

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
I suggest that the editor should have a way to reverse color offsets, just like in image editing software.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

Picture from https://graphicdesign.stackexchange.com/a/57225.

If this enhancement will not be used often, can it be worked around with a few lines of script?:
Trivial (but painful) for black and white gradients by switching leftmost and rightmost grabbers manually, not trivial for anything else.

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

[Calinou]

I opened a pull request for this, but it needs to be redone to use a context menu option instead of the middle mouse button.

[Chaosus]

Fixed by godotengine/godot#53321