feat: add stacks

Author: ayeo127

packages/panda-san/src/variables/index.scss

@@ -2,3 +2,4 @@
 @import 'colors';
 @import 'container';
 @import 'font';
+@import 'stacks';

packages/panda-san/src/variables/stacks.scss

@@ -0,0 +1,5 @@
+$stack-page: 0;
+$stack-sticky: 1;
+$stack-overlay: 2;
+$stack-modal: 3;
+$stack-notification: 4;