fix#1567
Previously multiple `v-on` handlers with different event attach option
modifers (`.once`, `.capture` and `.passive`) are generated as an array
of objects in the form of `[{ handler, options }]` - however, this
makes it pretty complex for `runtime-dom` to properly handle all
possible value permutations, as each handler may need to be attached
with different options.
With this commit, they are now generated as event props with different
keys - e.g. `v-on:click.capture` is now generated as a prop named
`onClick.capture`. This allows them to be patched as separate props
which makes the runtime handling much simpler.
BREAKING CHANGE: this.$emit() and setupContext.emit() no longer
return values. For logic that relies on return value of listeners,
the listener should be declared as an `onXXX` prop and be called
directly. This still allows the parent component to pass in
a handler using `v-on`, since `v-on:foo` internally compiles
to `onFoo`.
ref: https://github.com/vuejs/rfcs/pull/16
