Add a -vet-empty-case to help with switch bugs

[virror]

The problem:
A lot of people including my self seem to port projects and/or are used to other languages where case fallthrough is the default behavior for switch statements. This translates badly to Odin and easily causes bugs in the code when things like:
case: a
case: b
should instead look like:
case a, b:

The solution:
One solution would be to add a -vet-empty-case to throw errors when having empty cases and instead promote using a break when an empty case is intended. This would both help a lot against bugs but also increase the readability of the code by more clearly showing the intent.

This has been discussed briefly in the Discussion discord and got positive feedback.