Add "self" to position-area and position-try-fallbacks

Fixes w3c#13738

Author: josepharhar

css-forms-1/Overview.bs

@@ -1172,15 +1172,15 @@ select::picker(select) {
   max-block-size: stretch;
   overflow: auto;
   /* Below and span-right, by default. */
-  position-area: block-end span-inline-end;
+  position-area: self-block-end span-self-inline-end;
   position-try-order: most-block-size;
   position-try-fallbacks:
     /* First try above and span-right. */
-    block-start span-inline-end,
+    self-block-start span-self-inline-end,
     /* Then below but span-left. */
-    block-end span-inline-start,
+    self-block-end span-self-inline-start,
     /* Then above and span-left. */
-    block-start span-inline-start;
+    self-block-start span-self-inline-start;
 }
 ```