Support other phx- attribute

https://github.com/ArthurClemens/primer_live/blob/0526e29456cda146f1f9adcc6505d6ffe428b3cf/lib/helpers/declaration_helpers.ex#L38-L42 I need `phx-target` and `phx-value-*`, but it cause `undefined attribute "phx-target" in slot "item" for component...`. Not sure it is possible to extend from user-side.