Create composables for the MR Widget

Summary

Refactor the MR Widget to include composables to reduce the complexity in the ready_to_merge.vue component

Improvements

Ideally, we should reduce the current amount of business logic inside the ready_to_merge.vue component to multiple smaller components with clear responsibilities so the code is easier to navigate and maintain.

Risks

Involved components

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

Optional: Intended side effects

The MR Widget should behave the same as the current implementation.

Optional: Missing test coverage