Hidden/private signals

[Bromeon]

In Godot, prepending a signal with _ means the signal is hidden and won't be advertised in editor docs or the GDExtension JSON.

See:

• Don't expose underscored signals godotengine/godot#112770
• Hide signals prefixed by underscore godotengine/godot#115199

We need to see what this means for us, in particular:

1. For engine/non-Rust signals, the code generator should not create a signals()._hidden_signal() symbol.
   • If these are never included in extension_api.json, this may already be fulfilled.
2. When registering Rust signals, it should be possible to hide them from Godot.
   • Either implicit _ prefix, but I don't like that as a convention, as in Rust it means "unused" rather than "private".
   • Or explicit #[signal(hidden)], #[signal(priv)], #[signal(internal)] or so. The latter has prior art in #[class(internal)].
3. Document this in #[godot_api].