{:only} should also work on single declarations

[MikaelMayer]

As discussed as a team, the following should report only 3 errors instead of 6:

method {:only} TestReported() {
  assert false; // An error here
}

method TestNotReported() {
  assert false; //  No error here
}

class A {
  method {:only} TestReported() {
    assert false; // An error here
  }
  
  method TestNotReported() {
    assert false; //  No error here
  }
}

module B {
  method {:only} TestReported() {
    assert false; // An error here
  }
  
  method TestNotReported() {
    assert false; //  No error here
  }
}

That feature will make the use of {:only} on part with selective verification where users can verify only one method at a time, except that it would work for any IDE and also on the command line, without having to specify a specific attribute.