[Structured list] Add feature flag

[laurenmrice]

Acceptance criteria

• [x] Note: Start this issue once dev has a PR up for it.

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• [x] Create Feature flag boolean for the feature flag change.
  • Naming convention for feature flag boolean: "[name of enhancement](feature flag)".
  • Consider adding an icon before the name for easier scanning of feature flags present in the properties panel. (For example like ⛳ or any other icon that would make sense.)
• [x] Add a feature flag section in the "Description" for the Structured list component.
• [x] Review
  • [x] Lauren
  • [x] Juan

[thyhmdo]

After this is done, we can tackle on this issue https://github.com/carbon-design-system/carbon-design-kit/issues/752

[juanencalada]

@Kritvi-bhatia17 @laurenmrice just reviewed and good to merge