feat(build): add index files to bundles

[Thomaash]

This allows more visually appealing imports when used with bundlers:

// Old, but still works:
import * as vis from "vis-network/standalone/esm/vis-network";

// New, achieving the same result (ESM):
import * as vis from "vis-network/standalone";

// ESM explicitly:
import * as vis from "vis-network/standalone/esm";

// UMD explicitly:
import * as vis from "vis-network/standalone/umd";

This partially works in HTML but the old way is preferred (less requests,
library name in dev tools and better compatibility):

<!-- Old, still works. Recommended. -->
<script
  type="text/javascript"
  src="https://unpkg.com/vis-network/standalone/umd/vis-network.min.js"
></script>

<!-- New, works but has issues. Not recommended. -->
<script
  type="module"
  src="https://unpkg.com/vis-network/standalone/esm"
></script>

<!-- New, doesn't work. -->
<script
  type="text/javascript"
  src="https://unpkg.com/vis-network/standalone/umd"
></script>

[vis-bot]

🎉 This PR is included in version 6.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀