Improve wgsl lexer error messages.

Better errors for Unexpected, BadInteger, BadFloat, BadTexture, BadTypeCast, UnknownScalarType, UnknownStorageClass, UnknownAttribute, UnknownBuiltin, UnknownShaderStage, UnknownStorageFormat and UnknownConservativeDepth, ZeroStride, ZeroSizeOrAlign and UnknownType.

Also adds lexer::capture_span.

Also fixes some validation for texture sample types and and issue that cauld cause e.g. the type `f33` to be parsed as `f32`.