Better typehint for `FieldInterface::new(..., $label)`

[Lustmored]

Working on a project I noticed that FieldInterface::new method has $label typehinted as ?string, while all fields support TranslatableInterface|string|false|null (per Dto).

Changing it would be a BC change, but I suggest keeping it commented (as in other parts) so that it won't be missed during major bump.