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>

docs/app.vue

@@ -9,8 +9,12 @@
 </template>
 <style>
 @import 'assets/index.css';
+/* these are necessary styles for vue flow */
 @import 'node_modules/@braks/vue-flow/dist/style.css';
 
+/* this contains the default theme, you can skip it if you want to */
+@import 'node_modules/@braks/vue-flow/dist/theme-default.css';
+
 #app {
   height: calc(100% - 92px);
 }