Allow bit_set as type of bit_field's field

[LeonardoTemperanza]

There are situations where one would want to do this:

Flag :: enum { Flag_0, Flag_1 }
Flags :: bit_set[Flag, u32]
Test :: struct
{
    using _: bit_field u32 {
        some_value: u32 | 24,
        some_flags: Flags | 8,
    },
    other_field: u32
}

But this is currently not allowed, as "The type of a bit_field's field must be an integer, enum, or boolean".

One such place where this pattern is used is in the Vulkan bindings:

AccelerationStructureInstanceKHR :: struct {
	transform:                      TransformMatrixKHR,
	using _: bit_field u32 {
		instanceCustomIndex: u32 | 24,
		mask:                u32 | 8,
	},
	using _: bit_field u32 {
		instanceShaderBindingTableRecordOffset: u32                      | 24,
		flags:                                  GeometryInstanceFlagKHR  | 8,
	},
	accelerationStructureReference: u64,
}

Since it's currently not possible to include a bit_set in a bit_field, the bindings generator resorts to using the enum variant, resulting in rather unintuitive behavior since usage code needs to cast from GeometryInstanceFlagsKHR to GeometryInstanceFlagKHR.