[css-animations-2] Add play-once animation-action w3c#12611

Author: tabatkins

css-animations-2/Overview.bs

@@ -766,6 +766,7 @@ and what effect they have in each animation state:
     </thead>
     <tr><td><dfn>none</dfn><td>—<td>—<td>—<td>—<td>—
     <tr><td><dfn>play</dfn><td>—<td>{{play()}}<td>—<td>{{play()}}<td>{{play()}}
+    <tr><td><dfn>play-once</dfn><td>—<td>{{play()}}<td>—<td>{{play()}}<td>—
     <tr><td><dfn>play-forwards</dfn><td>set playback rate to positive<td>{{play()}}<td>—<td>{{play()}}<td>{{play()}}
     <tr><td><dfn>play-backwards</dfn><td>set playback rate to negative<td>{{play()}}<td>—<td>{{play()}}<td>{{play()}}
     <tr><td><dfn>pause</dfn><td>—<td>—<td>{{pause()}}<td>—<td>—