types(runtime-core): provide valid type for default $emit

[pikax]

relates to vuejs/test-utils#144

When using ComponentPublicApi the emit type defaults to an empty object, making the $emit to be unknown type instead of the default emit function (event: string, ...args: any[]) => void

[haoqunjiang]

This change seems to have broken vue-class-component.
Cc: @ktsn

[pikax]

@sodatea Thank you :)

There's the fix: #1584