CSS Nesting: enable preview feature flag

https://bugs.webkit.org/show_bug.cgi?id=250233
rdar://103967616

Reviewed by Tim Nguyen.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:

Canonical link: https://commits.webkit.org/258564@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -965,16 +965,16 @@ CSSMotionPathEnabled:
 
 CSSNestingEnabled:
   type: bool
-  status: testable
+  status: preview
   humanReadableName: "CSS Nesting"
   humanReadableDescription: "Enable support for CSS nesting https://www.w3.org/TR/css-nesting-1/"
   defaultValue:
     WebKitLegacy:
-      default: false
+      default: true
     WebKit:
-      default: false
+      default: true
     WebCore:
-      default: false
+      default: true
 
 CSSOMViewScrollingAPIEnabled:
   type: bool