Refactor affects_lightmapped_meshes, introduce AmbientLightFlags

[coreh]

Objective

• Make ambient light more consistent with other light sources in the code
• Pave the way for more ambient light boolean flags other than affects_lightmapped_meshes
• Make it easier to support monochromatic ambient lights as a follow up once Minimum Viable Monochromatic Lights #24428 lands

Solution

• Add AmbientLightFlags bitflags struct
• Rename ambient_light_affects_lightmapped_meshes to ambient_light_flags in GpuLights struct and matching WGSL-side Lights struct
• Added WGSL AMBIENT_LIGHT_FLAGS_AFFECTS_LIGHTMAPPED_MESHES_BIT constant
• Set up plumbing to both pass and use the new bitfield consistently with other light flags

Testing

• Did you test these changes? If so, how? Yes, by viewing the lighting and lightmaps examples
• Are there any parts that need more testing? Not that I know of
• How can other people (reviewers) test your changes? Is there anything specific they need to know? Manually verify ambient light still works
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test? macOS

[olukowski]

Code looks good, and ambient light still works properly in the lighting example on my machine.