Package json cleanup

[sdadn]

Summary

Additional Details

This PR cleans up package.json by:

• Enabling automatic peer Dependency installation.
• Removing all dependencies that aren't used directly within this repo. All those dependencies are installed automatically as the peer dependencies of terra components such as react, react-intl etc.
• Removing all terra components. Adding them was redundant as they were automatically installed as sub dependencies of the docs packages.
• Using the prettifyJSON script script to format the package.json file.
• Deleting the unused Heroku scripts.

This PR also drops npm 6 support as it now relies on automatic peer dependency installation.

Testing

This was tested by doing a clean install after removing dependencies and running npm start to ensure that the dev site builds and has the same content.

Thank you for contributing to Terra.
@cerner/terra

[adoroshk]

Tested locally by running npm run clean:install and npm run start commands. Workes as expected.