Add target-spacing rule

[WilcoFiers]

I wrote a load of test cases on how to do this:

https://codepen.io/wilcofiers/pen/abZxPow

I'd like to implement this rule while WCAG 2.2 is still in draft, so that it can be used for testing. We can take it out of "experimental" as soon as WCAG 2.2 becomes a recommendation.

[padmavemulapati]

@straker , can you please give QA notes to validate this ticket

[padmavemulapati]

Validated with the latest axe-core develop branch code base,
test script for the failure in target-size rule:

<!-- Failed examples -->
  <p>
    <button id="failCase1">x</button
    ><span role="button" tabindex="0" id="failCase2">x</span>
  </p>
  
  <ul>
    <li><a href="#" id="failCase3">Item1</a></li>
    <li><a href="#" id="failCase4">Item2</a></li>
  </ul>
  
  <p>
    <input type="checkbox" id="failCase5" /><input type="checkbox" id="failCase6" /><input
      type="text"
      id="pass-adjacent"
    />
  </p>