sibling-count() and sibling-index() are supported in STP (#28140)

* sibling-index() is now supported in STP

* sibling-count() is now supported in STP

Author: nt1m

css/types/sibling-count.json

@@ -24,8 +24,8 @@
             "opera": "mirror",
             "opera_android": "mirror",
             "safari": {
-              "version_added": false,
-              "impl_url": "https://webkit.org/b/294895"
+              "version_added": "preview",
+              "impl_url": "https://webkit.org/b/290698"
             },
             "safari_ios": "mirror",
             "samsunginternet_android": "mirror",

css/types/sibling-index.json

@@ -24,8 +24,8 @@
             "opera": "mirror",
             "opera_android": "mirror",
             "safari": {
-              "version_added": false,
-              "impl_url": "https://webkit.org/b/294895"
+              "version_added": "preview",
+              "impl_url": "https://webkit.org/b/290698"
             },
             "safari_ios": "mirror",
             "samsunginternet_android": "mirror",