[minor] Scrip tags manager

[lizkenyon]

What this PR does

• This PR adds the a script tag manager to the shopify_app gem.

• This functionality was removed in version 22 of this package.

• Script tags are still a valid way for modify the content of vintage themes.

• This PR has several differences between the previous script tag manager

  • Not the manager adds script tags with GraphQL instead of REST
  • This adds the ability to specify whether or not the script tags should be cached
  • You can now specify a template to check for before adding the script tag
    • Script tags should not be used if the theme has a valid template
  • Change the name from scripttag_manager to script_tag_manager

script_tag-720.mov

Reviewer's guide to testing

Things to focus on

1. Is the documentation clear?
2. Has functionality been restored?

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

[sle-c]

some minor comments, otherwise LGTM