feat: add flex modifiers GH-25

Author: RickEyre

packages/panda-san/src/layout/flex.scss

@@ -0,0 +1,8 @@
+.is-flex {
+  display: flex;
+
+  &--centered {
+    align-items: center;
+    justify-content: center;
+  }
+}

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

@@ -1,3 +1,4 @@
 @import 'container';
+@import 'flex';
 @import 'grid';
 @import 'level';