Publish pre-build bundles to npm (idiomatic npm life cycle script)

[bershanskiy]

This PR splits up portion of #7398 to simplify review.

This PR does the following:

• introduces prepack script that gets triggered automatically on npm publish. This script creates data bundle dist/data.json for npm release.
• introduces new main script dist/index.js that automatically chooses between index.js (for local development and testing) and dist/data.json (for downloads from npm)
• moves extend from actual dependency to devDependencies because it's not needed after bundling

This PR does not:

• remove package.json property engines
  I'd really like to see
• add tests
  Where would we even plug tests with the limited flexibility of npm life cycle scripts? What would they test?

Background:
#7374

A checklist to help your pull request get merged faster:

• Summarize your changes
• Data: link to resources that verify support information (such as browser's docs, changelogs, source control, bug trackers, and tests)
• Data: if you tested something, describe how you tested with details like browser and version
• Review the results of the linter and fix problems reported (If you need help, please ask in a comment!)
• Link to related issues or pull requests, if any

[ddbeck]

We decided to go the route of #7398, so I'm going to close this PR. Thank you this one too, @bershanskiy. It was genuinely instructive for landing the other PR. 🙇