Create IDE mirror modules and Vuex plugin

For now, the mirror is mostly stubbed.
It will be implemented in [this issue][1].

[1]: https://gitlab.com/gitlab-org/gitlab-ee/issues/5276

ee/app/assets/javascripts/ide/lib/mirror.js

+import createDiff from './create_diff';
+
+export const createMirror = () => {
+  const uploadDiff = () => {
+    // For now, this is a placeholder.
+    // It will be implemented in https://gitlab.com/gitlab-org/gitlab-ee/issues/5276
+  };
+
+  return {
+    upload(state) {
+      uploadDiff(createDiff(state));
+    },
+  };
+};
+
+export default createMirror();

ee/app/assets/javascripts/ide/stores/plugins/mirror.js

+import eventHub from '~/ide/eventhub';
+import mirror from '../../lib/mirror';
+
+export default function createMirrorPlugin() {
+  return store => {
+    eventHub.$on('editor.save', () => mirror.upload(store.state));
+  };
+}

ee/spec/frontend/ide/stores/plugins/mirror_spec.js

+import eventHub from '~/ide/eventhub';
+import { createStore } from '~/ide/stores';
+import createMirrorPlugin from 'ee/ide/stores/plugins/mirror';
+import mirror from 'ee/ide/lib/mirror';
+
+jest.mock('ee/ide/lib/mirror');
+
+describe('EE IDE stores/plugins/mirror', () => {
+  let store;
+  let plugin;
+
+  beforeEach(() => {
+    store = createStore();
+    plugin = createMirrorPlugin();
+
+    plugin(store);
+  });
+
+  it('does not initally call upload', () => {
+    expect(mirror.upload).not.toHaveBeenCalled();
+  });
+
+  it('uploads on editor.save event', () => {
+    eventHub.$emit('editor.save');
+
+    expect(mirror.upload).toHaveBeenCalledWith(store.state);
+  });
+});