[Theme Checks] Ensuring importmap is not being used on script tags in theme app extensions

[AribaRajput]

What are you adding in this PR?

As of Jan 6th this PR will not be shipping until more clarification is sought on whether or not this needs to be a SFR fix rather than a theme check due to browsers supporting multiple importmaps in the future - see discussion in issue below for more clarification.

Part 1 of #647

This PR tackles adding in a theme check for theme app extensions that will ensure that users are not adding script tags in theme app extensions with type importmap. The reason this is split into multiple PRs is to keep the theme check focused as the other portion will target type module and only on app embeds that target compliance_head.

What's next? Any followup issues?

Part 2 of #647 that will target app embeds that target compliance_head.

What did you learn?

What importmaps are 💃

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • It's in the allChecks array in src/checks/index.ts
  • I ran yarn build and committed the updated configuration files
    • If applicable, I've updated the theme-app-extension.yml config

• I included a minor bump changeset
• My feature is backward compatible

• I included a patch bump changeset