Action Cable's JavaScript library can optionally be imported as an ES6 module via `import { … } from "@rails/actioncable/src"`, but that module is broken in most of the releases published on npm:

```
ERROR in ./node_modules/@rails/actioncable/src/connection.js
Module not found: Error: Can't resolve './internal' in './node_modules/@rails/actioncable/src'
 @ ./node_modules/@rails/actioncable/src/connection.js
 @ ./node_modules/@rails/actioncable/src/index.js
```

Because `internal.js` was gitignored, it would only be included if the publisher happened to have it generated locally. Committing it to version control ensures that won't happen, and gives us better visibility into changes over time.

References:
- https://github.com/rails/rails/pull/34370
- c0368ad0