Add global isMobile computed util

open-isle-cli/src/utils/screen.js

@@ -0,0 +1,12 @@
+import { ref, computed } from 'vue'
+
+// reactive width value to watch window resize events
+const width = ref(window.innerWidth)
+
+// update width on resize
+window.addEventListener('resize', () => {
+  width.value = window.innerWidth
+})
+
+// global computed property
+export const isMobile = computed(() => width.value <= 768)