feat: merge store with props

* if options are passed to useVueFlow, we want to merge them with possibly passed props
* renamed flowStore to stateStore
* fix examples
* add store prop to pass an already existing store to flow

Signed-off-by: Braks <78412429+bcakmakoglu@users.noreply.github.com>

examples/Basic/Basic.vue

@@ -13,6 +13,7 @@ import {
   isNode,
   removeElements,
   Node,
+  useVueFlow,
 } from '~/index'
 
 const onNodeDragStop = ({ node }: { node: Node }) => console.log('drag stop', node)
@@ -54,6 +55,7 @@ const toggleclasss = () => {
     return el
   })
 }
+useVueFlow()
 </script>
 <template>
   <VueFlow