Create work items relationship widget shell

What does this MR do and why?

⚠ Note: This MR is part of the preparation work required for &7459 (closed), where we're extracting parts of the Hierarchy Widget frontend such that it can be re-used for Linked Work Items app.

Task	Merge request	Status
Extract Work item contents and create a wrapper	!128088 (merged)	Merged
Extract work_items_token_input.vue from app/assets/javascripts/work_items/components/work_item_links/work_item_links_form.vue	!128973 (merged)	Merged
Create work items relationship shell	!130456 (merged)	This MR
Create a work_item_relationship component in the work_item_relationship folder which lists the linked items based on FF checks	!130194 (merged)	On hold
Create a work_item_relationship_form component in the work_item_relationship folder which allows to select relationship type, paste link, show autocompletions based on license and FF checks
Allow removing the linked item
Work with UX to reposition metadata for both Hierarchy and Relationship widgets

Screenshots or screen recordings

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs.
2. Enable the feature flag Feature.enable(:linked_work_items)

Steps to verify:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page
5. Check the Linked items section
6. It should show a panel with a small message

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 #416558 (closed)