Create store for a11y MR widget

What does this MR do?

This MR is part of a larger issue to create the accessibility MR widget. Currently this is still dead code, so I'm not including a changelog.

This MR is creating the store for the MR widget. Part of that responsibility is parsing and comparing the base accessibility report with the head accessibility report received from the backend. Eventually this comparison will be moved to the backend, but it has been decided that for now we will have it done on the frontend.

The comparison is used to check for any new, resolved, or unresolved changes in the accessibility report since the base report was created. In the code, this is done through using lodash's intersection and difference methods.

Related to #39425 (closed)

Screenshots

No screenshots included since this is non-user facing code.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team