fix(types): emit type in SetupContext

[DerAlbertCom]

If you define a component with emits, then you can type this.$emits

So with this, you get autocompletion and signature validation on this.$emit

  defineComponent({
    emits: {
      click: (n: number) => typeof n === 'number',
      input: (b: string) => b.length > 1,
    },
    created() {
      this.$emit('click', 1)
      this.$emit('input', 'foo')
    },
  })

but you don't get the same behavior in the setup function.

  defineComponent({
    emits: {
      click: (n: number) => typeof n === 'number',
      input: (b: string) => b.length > 1,
    },
    setup(props, { emit ) {
      emit('click', 1)
      emit('input', 'foo')
    },
  })

this will work, gives no errors, but also no auto completion or compilation safety with typescript.

This PR is will enable that, so that the emit property of the SetupContext has typed emits.

I don't know if there is a reason why this is not already the behavior of SetupContext, but it seems to be a good addition.