Draft: Add user setting to toggle between legacy and new Web IDE

Heads up! This MR will always be in Draft: and smaller and more polished MR's will be spun out of this branch. When all the parts are merged, we will close this MR.

• Draft: Add user setting to toggle between legac... (!98295 - closed)
• feat(css): Add gl-md-ml-auto (gitlab-ui!3040 - merged)
• Fix flash margin in Web IDE (!98943 - merged)
• Part 1 - Hide top nav with vscode_web_ide featu... (!98891 - merged)
• Part 2 - Add "use_legacy_web_ide" to "user_pref... (!98945 - merged)
• Part 3 - Add alert to switch between legacy and... (!100488 - closed)
• Part 3 - Add "Switch to new Web IDE" in legacy ... (!102146 - merged)

What does this MR do and why?

This is the Draft MR of #370139 (closed).

Screenshots or screen recordings (2022-09-18)

With Feature.disable(:vscode_web_ide)

No changes 🎉

With Feature.enable(:vscode_web_ide)

Demo	Dark mode	Error while updating
20220918_demo