[spirv] Add spv.Variable

This is a direct modelling of SPIR-V's OpVariable. The custom assembly format
parsers/prints descriptor in a nicer way if presents. There are other common
decorations that can appear on variables like builtin, which can be supported
later.

This CL additionally deduplicates the parser/printer/verifier declaration
in op definitions by adding defaults to SPV_Op base.
by adding

PiperOrigin-RevId: 253828254