Value needs to be able to be checked against current value during morph

In Datastar you can do 2 way binding of inputs/select/textarea/etc. The incoming morphs may not have there value's set and idiomorph is treating that as a clearing of the value.

Per a conversation with @botandrose we need more detail in `attributeChangedCallback`. Specifically access to the target/incomings element, attrtibute name and possibly values if they are already in memory.

Taking inspiration from [Web Components](
https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_custom_elements#responding_to_attribute_changes), `(attrName:string, oldValue:T, newValue:T, targetEl:Element, incomingEl:Element) ` seems to be a decent signature for the callback.