TS: allow strong typing of directive value

[jods4]

What problem does this feature solve?

Directive<T> is generic in the type of target element.
It would be nice to have a second generic value (can default to any) for the binding.value type.

So that this can be written:

interface Interval {
  from: number;
  to: number;
}

const dir: Directive<Element, Interval> = (el, { value: { from, to } }) => {
  // from, to are number
}

Without this, you have to cast binding.value explicitely inside the function body.

Hopefully this could be used in the future by Vetur to validate proper directive usage in templates.

What does the proposed API look like?

Modify to Directive<TElement = any, TValue = any>