Introduce basic version of Monaco implementation to be used in single-file editing scenarios

Introduce the barebones version of the Monaco editor's integration into GitLab to deliver the bare minimum for single-file editing experience.

Why not to use the existing app/assets/javascripts/ide/lib/editor.js?

Because it is way too feature-heavy and too WebIDE-coupled for a single-file use case. See the Epic for more details.