Default model value

This adds the ability to set a default value for the modelValue prop, which primarily would be used for detecting when a prop is absent since Vue changed this behavior in v3.1.

New API

This release completely changes the API with two goals in mind:

• Make it much easier to remember - now there's only really 2 functions to keep in mind regardless of the type of model you're creating
• Enable built-in modifiers to be used in Vue 3, this was recently patched in, but requires emit instead of directly calling the update-function-prop