Update starter_theme name to source_theme_name

Related issue

Closes #1

Summary

Changes only impact new theme generation so the changes are non-breaking. I updated README instructions with futher detail and updated the starter_theme machine name.

Breaking change

Indicate if this update includes a breaking change by selecting an option below:

• This IS NOT a breaking change.

Testing and review

Setup

1. Insert any required steps to take before beginning test.

QA/Test

1. Insert steps to test and confirm the result resolves the issue.

Checklist for the Developer

• A link to the issue has been included above.
• No merge conflicts exist with the target branch.
• Automated tests have passed on this PR.
• Deployment and testing steps have been documented above, if applicable.

Checklist for the Reviewers

• The file changes are relevant to the task objective and resolve the issue.
• Code standards and best practices are followed.
• QA/Test steps were successfully completed.
• Applicable logs are free of errors.