Support `monaco-editor` in Jest

Description

If you try migrating the spec/javascripts/ide/components/repo_editor_spec.js to Jest, you'll quickly learn that monaco-editor is incompatible with Jest...

Test suite failed to run

    Cannot find module 'monaco-editor' from 'editor_spec.js'

    > 1 | import { editor as monacoEditor } from 'monaco-editor';
        | ^
      2 | import Editor from '~/ide/lib/editor';
      3 | import { file } from '../helpers';
      4 | 

      at Resolver.resolveModule (node_modules/jest-resolve/build/index.js:230:17)
      at Object.<anonymous> (spec/frontend/ide/lib/editor_spec.js:1:1)

This is probably related to us having a special Webpack loader for monaco...

Proposal

This comment implied that this might not be an issue with newer versions of monaco. If that doesn't work, we'll need to look at building a mock 😞

UPDATE: We are past the version mentioned in the linked comment above, but are still having issues. See details in this note.

Context

The following discussion from !22095 (merged) should be addressed:

• @pslaughter started a discussion:

  note: Had to revert migrating the following specs:

  spec/javascripts/ide/components/activity_bar_spec.js
  spec/javascripts/ide/components/ide_tree_list_spec.js
  spec/javascripts/ide/components/repo_tab_spec.js

  Because for some reason the combination of specs + Karma's non-sandboxed environment caused repo_editor_spec to fail with some weird errors:

  Error: ModelService: Cannot add model because it already exists!
  9644           at ModelServiceImpl.../../../node_modules/monaco-editor/esm/vs/editor/common/services/modelServiceImpl.js.ModelServiceImpl._createModelData (webpack:///builds/gitlab-org/gitlab/node_modules/monaco-editor/esm/vs/editor/common/services/modelServiceImpl.js:382:1 <- spec/javascripts/test_bundle.js:1535116:19)
  9645           at ModelServiceImpl.../../../node_modules/monaco-editor/esm/vs/editor/common/services/modelServiceImpl.js.ModelServiceImpl.createModel (webpack:///builds/gitlab-org/gitlab/node_modules/monaco-editor/esm/vs/editor/common/services/modelServiceImpl.js:392:1 <- spec/javascripts/test_bundle.js:1535126:30)
  9646           at doCreateModel (webpack:///builds/gitlab-org/gitlab/node_modules/monaco-editor/esm/vs/editor/standalone/browser/standaloneEditor.js:82:1 <- spec/javascripts/test_bundle.js:1574530:102)
  9647           at Object.createModel (webpack:///builds/gitlab-org/gitlab/node_modules/monaco-editor/esm/vs/editor/standalone/browser/standaloneEditor.js:97:1 <- spec/javascripts/test_bundle.js:1574545:16)
  9648           at new Model (webpack:///ide/lib/common/model.js:1132:1 <- spec/javascripts/test_bundle.js:2202524:69)
  9649           at ModelManager.addModel (webpack:///ide/lib/common/model_manager.js:460:1 <- spec/javascripts/test_bundle.js:2203416:42)
  9650           at UserContext.<anonymous> (webpack:///builds/gitlab-org/gitlab/spec/javascripts/ide/lib/common/model_manager_spec.js:79:1 <- spec/javascripts/test_bu

  Creating a follow up issue for this...