GDShader: Add hint_enum for uniform int

[SomeRanDev]

This adds a new shader uniform hint for int named hint_enum.

uniform int noise_type : hint_enum("OpenSimplex2", "Cellular", "Perlin", "Value") = 0;

Similar to @export_enum, this changes the int input into a dropdown in the editor.

This has been requested in #8738 and then later in godot-proposals #7847

Unresolved Issues

There are two unresolved aspects to this PR:

• This PR adds strings to the .gdshader parser, and maybe the error handling could be better? At the moment, using strings outside of this hint results in something like: Expected expression, found 'STRING_CONSTANT'. Maybe it should be something more coherent, like: Strings are not supported in shaders.?
• While the visual shader implementation of this technically works, there appears to be some editor UI issues with visual shader nodes using PackedStringArray and String inputs. I wasn't able to figure out how to fix this.

---

• Bugsquad edit, closes: Add hint_enum to int shader uniforms godot-proposals#7847

[Chaosus]

I think you should recreate it as a separate branch rather than pushing a master - this is a common way to contribute to the engine, to avoid problems. You need to squash the commits also (see https://docs.godotengine.org/en/latest/contributing/workflow/pr_workflow.html#modifying-a-pull-request).

[SomeRanDev]

Ahh, sorry for my mistake! I didn't realize pushing from master would cause issues. I'll redo everything in a sec (+ squash commits from this point forward). 👌