Part 1 - Hide top nav with vscode_web_ide feature flag

• 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 "Switch to new Web IDE" in legacy ... (!102146 - merged)

What does this MR do and why?

In this MR, we support the UX goals #370139 (closed) and https://gitlab.com/gitlab-org/gitlab-web-ide/-/issues/25#note_1076057874 and hide the top nav when :vscode_web_ide feature flag is enabled.

Screenshots or screen recordings

Default

Desc	Before	After (this MR)
🎉 With Feature.disable(:vscode_web_ide), no changes
🎉 With Feature.enable(:vscode_web_ide) top nav is now hidden

Noisy

• With outdated_browser warning forced to true, system headers and footers, and a broadcast message.

Desc	Before	After (this MR)
🎉 With Feature.disable(:vscode_web_ide), no changes
🎉 With Feature.enable(:vscode_web_ide) top nav is now hidden

How to set up and validate locally

1. Visit the Web IDE for any public non-empty project.
2. Try out with both the feature flag :vscode_web_ide enabled and disabled.

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)