added version_added for FF anchor-size() (#27440)

Author: dletorey

css/types/anchor-size.json

@@ -16,8 +16,15 @@
             "chrome_android": "mirror",
             "edge": "mirror",
             "firefox": {
-              "version_added": false,
-              "impl_url": "https://bugzil.la/1838746"
+              "version_added": "preview",
+              "flags": [
+                {
+                  "type": "preference",
+                  "name": "layout.css.anchor-positioning.enabled",
+                  "value_to_set": "true"
+                }
+              ],
+              "impl_url": "https://bugzil.la/1972610"
             },
             "firefox_android": "mirror",
             "oculus": "mirror",
@@ -50,7 +57,15 @@
               "chrome_android": "mirror",
               "edge": "mirror",
               "firefox": {
-                "version_added": false
+                "version_added": "preview",
+                "flags": [
+                  {
+                    "type": "preference",
+                    "name": "layout.css.anchor-positioning.enabled",
+                    "value_to_set": "true"
+                  }
+                ],
+                "impl_url": "https://bugzil.la/1972610"
               },
               "firefox_android": "mirror",
               "oculus": "mirror",