Setter

This type represents a modifier that sets a "property" of an element.

It could be an html attribute, an event prop, or even a custom focus prop, the main constraint is that it should be idempotent, so that applying it several times in a row would produce the same effect as applying it once.

That way we can provide it to onMountSet { c => setter } and expect things to work if the element is mounted several times. However, note that CompositeKeySetter has special behaviour.