[FR] Function style declaration of defineEmits

[wenfangdu]

What problem does this feature solve?

Now that tuple style declaration is supported, can we also add function style declaration? It's closer to how we write defineProps<T>(), and unlike tuple labels, the arguments have to be named, therefore more constraints.

What does the proposed API look like?

const emit = defineEmits<{
  update(val: string): void
}>()

[yyx990803]

This has been discussed in the RFC repo: vuejs/rfcs#500