Add defineEmit() macro

[fisker]

Should be easier to use to emit in multiple places or forward an event.
The emit function returns from defineEmits() also confusing for beginners, I often saw them named it as emits.

- const emit = defineEmits(['foo'])
+ const emitFoo = defineEmit('foo')

const bar = () => {
-  emit('foo')
+  emitFoo()
}

<script setup>
- const emit = defineEmits(['foo'])
- const forwardFooEvent => (...arguments_) => emit('foo', ...arguments_)
+ const emitFoo = defineEmit('foo')
</script>

<template>
  <Child
-    @foo="forwardFooEvent"
+    @foo="emitFoo"
  />
</template>

Similarly, I also want defineOption(), but it can be a different proposal.