title: No such method/property 'isString' in Firebase Database rule
body: <p>I'm new in Firebase, I want each project property as string of minimum length 32</p>
  
  <pre><code>{
  "rules": {
    "project": {
      "$A": {
        ".read": true,
        ".write": "auth != null &amp;&amp; $A.isString() &amp;&amp; 32 &lt;= $A.length",
  </code></pre>
  
  <p>Here <code>$A</code> is the project property name <code>$A.isString()</code> is not aviliable &amp; getting error</p>
  
  <p><a href="https://i.stack.imgur.com/I6e7H.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/I6e7H.png" alt="enter image description here"></a></p>
  
  <p>How i check the <code>$A</code> is string of minimum 32 length ?</p>