zb: Split bus names type into a separate crate: zbus_names

As stated in the description of !341 (merged), it'd be great to check the names passed to the zbus_macros macros for conformance at compile-time but since zbus_macros can't depend on zbus itself, best we split all the names to a separate crate.

Since this will be an API-break, I'm doing this part already. Use in the macros will not require any API changes so we can do that later.