chore(shadertools): Support boolean shader defines only (v9.2)

[ibgreen]

For #

• Proposal for luma.gl API change, defines: Record<string, number, boolean> => defines: Record<string, boolean>

Background

• See discussion in shadertools: PBR module using UBO #2173 (comment)
• WGSL doesn't have a preprocessor, we have a simple #ifdef/#endif processor in shadertools, but no interest in making it more full featured (e.g. support text replacement, #if defined(...) etc.
• WGSL has @override constants for scalar values, that can be set at Pipeline creation "modifying" an already compiled shader. Suggest we add a new constants or overrides section for numeric values that takes advantage of this on WebGPU.
• For string valued defines, we already have an injection system, let's rely on that.

Change List

• defines can only have boolean values.