fix(css): add core.css (#15220)

fixes #15170

core/src/css/ionic.bundle.scss

@@ -0,0 +1,8 @@
+// Core CSS, always required
+@import "./core";
+
+// Global CSS: blocks scrolling, sets box-sizing
+@import "./global.bundle";
+
+// CSS Utils
+@import "./utils.bundle";