Add new vuex store for merge conflicts

What does this MR do?

This MR paves the way to refactor the data layer of the merge_conflicts app by adding a new vuex store based on:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/merge_conflicts/merge_conflict_store.js
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/merge_conflicts/merge_conflict_service.js

Since the original store has no test we are also introducing a set of basic tests for actions

Since the MR is already huge we will be adding more unit test in a followup MR where we will also hook the store to the components.

Plan

Description	MR
Add new store	👈 you are here
Add basic tests for Utility, getters and mutation	!56494 (merged)
Wire the new store in the components and remove old one	TBD

Screenshots (strongly suggested)

No visual changes in this MR: here is a video of the store at work when everything is wired

MergeRequestVuex

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this MR is a pure refactor.
• [-] 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

Related to 321090