Make scoped slot mechanism a public API

[justin-schroeder]

Describe the new feature you'd like
Currently vue-formulate uses scoped slots internally, but this API is not public. It would be great to have an optionally public API (well documented) for using it.

<FormulateInput>
  <template #label="{ label }">
    <label>I want my own label code!</label>
  </template>
</FormulateInput>

So technically the above code works in 2.0.0 already, but its not fully fleshed out and does not have access to the entire context object, we should give each slot access to the same internal context object which holds all the information about the component.

What percentage of vue-formulate users would benefit?
I hope not more than 40% because while scoped slots are super powerful, they require a much more in-depth understanding fo the inner-workings of vue-formulate and the goal is for this to be super easy to use. But we do need to be able to handle more complex use cases.