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>

postcss.config.js

@@ -1,4 +1,3 @@
 module.exports = {
-  inject: true,
   plugins: [require('autoprefixer'), require('postcss-nested')],
 }