fix(compiler-core): keep event handlers noninitial arguments

[cathrinevaage]

When the compiler encounters a template such as this

<AComponent @an-event="anEventHandler" />

the function passed as the prop onAnEvent to <AComponent> is generated like this

$event => _ctx.anEventHandler($event)

This becomes a problem when the event is emitted with more than one argument. Only the first argument, $event, is passed to the event handler.
This is especially evident when wanting to use the transition enter/leave's resolve function.

<script>
export default {
  setup: () => ({
    onEnter (el, resolve) {
      ...
      resolve() // Uncaught TypeError: resolve is not a function
    }
  })
}
</script>

<template>
  <transition @enter="onEnter">
    <... v-if="..." />
  </transition>
</template>

This PR fixes this by replacing $event with ($event, ...args) in code generation for the vOn transform.
Resulting in functions like ($event, ...args) => _ctx.eventHandler($event, ...args).

Note

This is my first pr to vuejs, and I have limited experience in working with tests. Although the tests are passing, I'm not convinced everything is correct.
When working to resolve this issue, this line was never touched. I can also see that the coverage test doesn't touch it either. I haven't quite understood what it does, but I decided to change it anyway.

Additional note

e2e kept failing when running the tests. Sometimes they passed, other times the failed. I don't know if this is common, or if it's related to my setup somehow. I've managed to at some point get all tests to pass, while running them individually.