[Part 4] 5276 Create terminal sync Vuex module and update plugin

What does this MR do?

This MR is part of the larger https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12239.

This MR creates the terminal_sync Vuex module and updates the corresponding Vuex plugin.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14035
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

Links

• Part of #5276 (closed)