feat: copy default theme css file separately to dist

* to enable overwriting the default theme export it separately
* add rollup plugin copy for copying the css file

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

src/container/Flow/Flow.vue

@@ -190,5 +190,4 @@ const edgeTypes = controlledComputed(
 </template>
 <style>
 @import '../../style.css';
-@import '../../theme-default.css';
 </style>