Require that classes starting with "Editor" be marked internal

[Bromeon]

Make this edge case explicit to users: create_dialog.cpp#L171-L173

Effect:

#[derive(GodotClass)]
#[class(init)]
pub struct EditorObject {}

---

The current PR does not allow registering classes starting with Editor in <4.2 at all, but we're anyway going to phase out Godot 4.1 for godot-rust v0.4.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1272