Introduce work_items_hierarchy feature flag

What does this MR do and why?

Hides component introduced in !88097 (merged) behind :work_items_hierarchy feature flag.

Slack conversation for context: https://gitlab.slack.com/archives/C02403NRD40/p1653436766819919?thread_ts=1653436384.678409&cid=C02403NRD40

Screenshots or screen recordings

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How to set up and validate locally

1. Enable work_items feature flag

   Feature.enable(:work_items)

2. Visit an issue page, and add a checklist item to description

3. Hover over checklist item, then click 'Convert to task' button on the left

4. Save the work item

5. Click on the newly created work item

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #362852 (closed)