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

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