Refactor EE IDE store extend into plugins

What does this MR do?

This MR refactors the ee/ide/stores/extend into "plugins" and a function that creates and applies them.

Why?

The Web IDE file mirroring will need to register a plugin, similar to the terminal plugin.

Since plugins are a native Vuex thing, it is helpful keeping the consistent terminology and interface (even though we aren't using the plugins property yet).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style 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.

Related

• https://gitlab.com/gitlab-org/gitlab-ee/issues/5276
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11478