feat: add isEditing composable

src/ui/composables/editor.js

@@ -0,0 +1,19 @@
+import { ref } from 'vue';
+
+import { useEventListener } from './event';
+
+/**
+ * @param {import('../../../openmct').OpenMCT} openmct
+ * @returns {{isEditing: import('vue').Ref<boolean>}} isEditing
+ */
+export function useIsEditing(openmct) {
+  const isEditing = ref(false);
+
+  // eslint-disable-next-line func-style
+  const handler = (value) => (isEditing.value = value);
+
+  // Use the base event listener composable
+  useEventListener(openmct.editor, 'isEditing', handler);
+
+  return { isEditing };
+}