Do not allow non class functions in a class file

Is your feature request related to a problem?

When a file contains a class, it should not allow any functions that are outside of this class.
Since WPCS requires renaming files to include the class name, I think it would be consistent to not allow any functions that are not part of this class to be in this file.

Describe the solution you'd like

file: class-my-test.php

class my_test {

}

function do_something() {
    echo "hello";
}

Should get flagged for do_something, as its the file for my_test

Search the PHP_CodeSniffer rules for the check that enforces class-file naming and trace how class and function declarations are inspected. Add coverage for a file containing a class plus an unrelated global function, with completion defined as that function being flagged while class members remain allowed.