feat: add package_json_key setting

[puehringer]

Developer Checklist (Definition of Done)

Issue

• All acceptance criteria from the issue are met
• Tested in latest Chrome/Firefox

UI/UX/Vis

• Requires UI/UX/Vis review
  • Reviewer(s) are notified (tag assignees)
  • Review has occurred (link to notes)
  • Feedback is included in this PR
  • Reviewer(s) approve of concept and design

Code

• Branch is up-to-date with the branch to be merged with, i.e., develop
• Code is cleaned up and formatted
• Unit tests are written (frontend/backend if applicable)
• Integration tests are written (if applicable)

PR

• Descriptive title for this pull request is provided (will be used for release notes later)
• Reviewer and assignees are defined
• Add type label (e.g., bug, feature) to this pull request
• Add release label (e.g., release: minor) to this PR following semver
• The PR is connected to the corresponding issue (via Closes #...)
• Summary of changes is written

Summary of changes

• Adds a package_json_key (default visyn) to enable multi-page builds from the same package.json, i.e. via visyn_scripts bundle --mode production --env workspace_mode=single package_json_key=my-custom-page

Screenshots

Additional notes for the reviewer(s)

Thanks for creating this pull request 🤗