no-unreachable-code: handle exhaustive switch statements

[ajafff]

function testSwitch(v: 1 | 2): number {
  switch (v) {
    case 1:
    case 2:
      return 1;
    default:
      console.log('default'); // unreachable
  }
  console.log('after'); // unreachable
}

The behavior for default should be configurable to allow for assertNever(v) calls.
Maybe we can even generalize it to allow never returning functions and throw statements.