Setup numeric precision specification for backends

[Shfty]

• Backends are currently fixed to f32 due to the need to inject literals
• Backends should have control over the final type
  • ex. naga backend should be able to downcast f64 to f32, enable the FLOAT64 capability and upcast everything to f64, or enable the FLOAT64 capability but otherwise leave types as-is to allow mix-matching in cases where it makes sense(??)
  • Need to be able to guarantee output type rather than relying on literals, while still allowing arbitrary storage for the sake of nice API
  • May be wiser to simply store as highest-common-denominator types
  • Specifying format per-property in SpecializationData is probably the way to go
  • Switch back from identifiers to properties to preserve type information
  • Augment with a new numeric type enum so backends can cast appropriately