[css-anchor-position] Add anchor-default and inset-area to the allowed props w3c#9950

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1310,6 +1310,8 @@ the following [=properties=]:
 * [=margin properties=]
 * [=sizing properties=]
 * [=self-alignment properties=]
+* 'anchor-default'
+* 'inset-area'
 
 Issue(#9195): What exactly are the constraints that determine what's allowed here?
 Current list is based off of what's reasonable