Fix: add aria-level as supported on treeitem (#1676)

closes #1605

adds `aria-level` to the supported states and property cell of the characteristics table since its no longer being inherited due to the change to `listitem`

Author: scottaohara

index.html

@@ -10002,6 +10002,7 @@ <h5>Note regarding the ARIA 1.1 <rref>none</rref> role.</h5>
 							<ul>
 								<li><sref>aria-expanded</sref></li>
 								<li><pref>aria-haspopup</pref></li>
+								<li><pref>aria-level</pref></li>
 							</ul>
 						</td>
 					</tr>