Adding tooltips for export variables

[VitaZheltyakov]

Describe the project you are working on:
Kinetic scrolling plugin

Describe the problem or limitation you are having in your project:
The absence of tips for export variables

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Add the ability to set a hint for export variables

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
I think that it will be most convenient to display the previous
comment as a hint for a export variable

If this enhancement will not be used often, can it be worked around with a few lines of script?:
No, It is Immpossible

Is there a reason why this should be core and not an add-on in the asset library?:
This feature improves the usability of export variables.

[Jummit]

Will be fixed by godotengine/godot#35716

[mhilbrunner]

Reopening until godotengine/godot#35716 is merged.