Remove _IS_NUMERIC macro in favor of concrete funtion CPIsNumeric()

- Create Foundation/_CPFoundationUtilities.j to house global low-level utilities.
- Implement CPIsNumeric as a concrete JavaScript function, preserving the exact legacy logic.
- Import _CPFoundationUtilities.j in the Foundation.j umbrella manifest.
- Replace _IS_NUMERIC macro calls at all call sites with CPIsNumeric().

AppKit/CPStackView.j

@@ -20,8 +20,6 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "../Foundation/Foundation.h"
-
 @import "CPView.j"
 
 // Gravity Areas