Shaders: node macro (#2923)

* node_macro: cleanup attr parsing

* node_macro: add `cfg()` attr to feature gate node impl

* node_macro: add `shader_nodes` option

* node_macro: fixup tests

node-graph/node-macro/src/lib.rs

@@ -5,6 +5,7 @@ use syn::GenericParam;
 mod codegen;
 mod derive_choice_type;
 mod parsing;
+mod shader_nodes;
 mod validation;
 
 /// Used to create a node definition.