Add second option for adding OpNames to whole module

[khyperia]

Right now, OpNames for the whole module is enabled by setting debuginfo to be enabled in cargo (passed on to rustc). This isn't very discoverable, and while docs should definitely be improved to make it be more discoverable, it'd be nice to add another option to enable OpNames for the whole module in spirv-builder - similar to how name_variables exists today. (both the spirv-builder and cargo config option would do the same thing)

(maybe change name_variables into a more generic sounding option, and turn it into an enum {None, Variables, Everything} or something, idk)