Add vue/require-emit-validator rule

[g-plane]

Fixes #1482 .

[ota-meshi]

Thank you for this PR! I have some change requests.

[g-plane]

I have a question about null for skipping validation: some new users may just want to define an event without any payload, and they may choose to use null, but actually they should use () => true which means "no payload". Should we detect for null and provide a suggestion which advises them to use () => true?

[ota-meshi]

Thank you for the question.
I think we need to suggest () => true instead of null. () => true can type check for no payload.
In addition, if the user wants to check for no payload, the user can also set (...args:[]) => args.length === 0.

[ota-meshi]

LGTM! Thank you!