add new lint: `unnecessary_breaks`

[pq]

From the spec:

this proposal removes the requirement that each non-empty case body definitely exit. Instead, a non-empty case body implicitly jumps to the end of the switch after completion.

We might consider linting on unnecessary breaks.

[bwilkerson]

Yes, I think we do.

[pq]

/fyi @munificent for 2 cents.