Consider renaming instance_id to instance_index.

[Vecvec]

Is your feature request related to a problem? Please describe.
instance_id is the index into the tlas package's instances member, however unlike most of the other indexes into things, this one is called *_id instead of *_index. This is likely because of instance_custom_index which is similar sounding, but it does not change any index so it's name is also confusing.

Describe the solution you'd like
We should rename instance_id to instance_index and instance_custom_index to instance_custom_data.

Describe alternatives you've considered
Not change their names.

Additional context
This currently matches the SPIRV spec (which is inconsistant with itself)