GDScript: Add custom export hints

[SirQuartz]

This pull request implements optional custom export hints when using @export.

Now you can do something like this:

@export("suffix:m") var myvar: Vector3

It'll appear like this in the inspector:

Here's a quick example:

custom.export.hint.mp4

In this example, I use "m" for meters as the exported suffix hint, but you could use anything as the hint like "g/cm^3", "mph", "gallons", "psi", etc.

Resolves: #61325

[vnen]

This seems to work only for a very specific case. I would rather add a @export_custom annotation that allows setting any hint and hint_string so it can be used to all cases that don't have a specific annotation type.

[YuriSizov]

Superseded by #72912.