Fix false positive errors in gdshaderinc files

[ze2j]

Fixes #89479

These language features are wrongly detected as not supported when used in gdshaderinc files:

• instance uniforms
• hint_normal_roughness_texture
• hint_depth_texture

These types of error should only occur when the shader type is not spatial and not a gdshaderinc file.

Here is the project I used to test this PR: test_project.zip
There is one scene per feature in their own folder + 1 canvas item shader for non regression tests.

As expected:

• When a spatial shader use one of these language feature, no error is raised in the spatial shader nor the gdshaderinc file it includes.
• An error is raised in the canvas item shader with or without including a gdshaderinc file.

[clayjohn]

Makes sense to me. Maybe @bitsawer could take a look as well

[akien-mga]

Tested with the MRP, this seems to work well, including when using those gdshaderinc files in canvas_item shaders.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉