Reorganize `meta`/`register` modules slightly

[Bromeon]

There is some unclear split of responsibilities:

• godot::meta

  • PropertyInfo
  • PropertyHintInfo
  • MethodInfo

• godot::register

  • property module with Var, Export, BuiltinExport, PhantomVar
  • Derive macros: Var, Export, GodotConvert, GodotClass
  • Attribute macros: godot_api, godot_dyn
  • Signals on top level: TypedSignal, ConnectBuilder, ConnectHandle, SignalReceiver, IndirectSignalReceiver
  • Misc: RpcConfig

It would probably make sense to:

1. Group signals in their own module.
2. Consider property-related symbols into one module (away from meta?)
3. Decide whether attribute/derive macros should be modularized based on macro-ness or based on their scope.
4. Remove more rarely used traits like Var/Export from prelude (they're usually derived, if at all). Also some builtin-related enums.

Adding register::signal would probably be quite straightforward, however it creates quite a deep nesting. For signals it's also not quite clear that they're part of register, because the functionality is shared between engine-provided signals, user-provided ones and several core mechanics (TypedSignals). This is also true to a lesser extend with register::properties once meta parts are moved in -- they contain Var parts which are clearly user-registration related, but PropertyInfo/ MethodInfo is rather part of Godot's type system. Maybe they should stay in meta?

Then there's also the problem that PropertyInfo/PropertyHintInfo/MethodInfo are in meta, but PropertyHint/PropertyUsageFlags are in global. They are almost always used in combination though, but the user then has to import from different modules. We already extract builtin-related enums to godot::builtin. Some of them like Side are accidentally in the prelude.

Tip

TLDR: What should guide module organization is primarily:
Which symbols does the user likely use together?

Less so how Godot exposes its APIs, or how things are named, or what simplifies implementation. UX first.