Add support for rollup watch mode

What does this MR do and why?

• Adds a yarn command for running Rollup in watch mode (https://rollupjs.org/command-line-interface/#w-watch)
• Enables cache for watch mode (https://rollupjs.org/configuration-options/#cache)
• Documents how to use watch mode and updates some other JavaScript-related documentation

If you're only working with frontend JavaScript code, using "rollup watch" allows for seeing compiled changes locally in ~5 seconds instead of ~40 seconds. Thanks to @eread for surfacing the docs on rollup cache. 🙌

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Checkout this branch.
2. Run yarn watch.
3. Modify a .js or .vue file within the /content/frontend directory.
4. Verify that rollup generates a new bundle for changed file.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.