Support multiple unnamed fields in relationship derive

[Azorlogh]

Objective

• Relationship derives support structs with multiple named fields thanks to the #[relationship] attribute.
• They don't however support structs with multiple unnamed fields, so this code would fail:

#[derive(Component, Debug)]
#[relationship(relationship_target = TargetedByGeneric<T>)]
struct Targeting<T: Send + Sync + 'static>(#[relationship] Entity, PhantomData<T>);

#[derive(Component, Debug)]
#[relationship_target(relationship = TargetingGeneric<T>)]
struct TargetedBy<T: Send + Sync + 'static>(#[relationship] Vec<Entity>, PhantomData<T>);

Solution

• Reuse the same logic as for named fields, but for unnamed fields

Testing

• Trivial

[alice-i-cecile]

Can you add a regression test please? This looks fine, but I want to make sure that it works and keeps working.

[Azorlogh]

Can you add a regression test please? This looks fine, but I want to make sure that it works and keeps working.

Oh right! Added 👌