Check before requesting to resolve icons

Though we had checked if the icon is fetching or failed in `resolve`, this
promised function gets called on each prop updates, creating many many
unneccessary Promise instances.

src/components/BSMap/BSIcon.vue

@@ -40,7 +40,7 @@ const parts = computed(() => {
 });
 
 watch(parts, ({ type, src }) => {
-  if (type == 'icon') {
+  if (type == 'icon' && typeof iconStore.icons[src] == 'undefined') {
     iconStore.resolve(src);
   }
 }, { immediate: true });