[WGSL] WGSL needs a plan for expansion #600

I expect WGSL will evolve over time to incorporate new functionality.

We should make a plan to accommodate that growth.

For example:

• Account for potential conflict between keywords introduced in a future version of WGSL, vs. identifiers used in today's shaders. Maybe use namespace mechanism?
• Account for new data types, e.g. IEEE 16bit float, potentially other exotic floating points type like bfloat16. This can impact type derivation / typechecking.
• Will we require shaders to declare their own version or extensions? This is one approach to accommodating the keyword-conflict issue. GLSL chose this path.