Add Draft merge widget check

What does this MR do and why?

For #425337 (closed)

Adds Storybook-only (for now) implementation of the Draft-status check for the unified MR checks widget.

Screenshots or screen recordings

This component did not exist before, and it currently only renders in Storybook, so this screenshot lacks a bit of context, but here ya go:

How to set up and validate locally

1. yarn storybook:build && yarn storybook:start
2. Click the "draft" heading under the vue_merge_request_widget merge_checks section

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.