Part 3 - Add alert to switch between legacy and new Web IDE

• Draft: Add user setting to toggle between legac... (!98295 - closed)
• ...
• 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 MR introduces a dismissible (but not persistently dismissible) alert to switch between the legacy and new Web IDE experience. This alert is only visible if the Feature Flag vscode_web_ide is enabled.

Screenshots or screen recordings

Demo	Dark mode	Error while updating
20220918_demo

How to set up and validate locally

1. Enable vscode_web_ide feature flag by opening a rails console rails c and running Feature.enable(:vscode_web_ide)
2. Visit the Web IDE for any non-empty project.
3. Click the links and "Switch to" button in the alert that appears at the top of the Web IDE.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #370139 (closed)