Add umd dist file

[twsl]

I'd like to include the project using an umd file, which is currently not present in the dist.
Adding different output formats would enable a variety of different usecases

[bcakmakoglu]

If you want to use VueFlow without a build tool like Vite/Webpack you can use it directly from jsdelivr: https://cdn.jsdelivr.net/npm/@vue-flow/[email protected]/dist/vue-flow-core.iife.min.js