Step 4 - Use gitlab-smooshpack behind feature flag

• POC
• Step 1 - Update IDE to track mimeType
• Step 2 - Use arraybuffer for binary files in Web IDE
• Step 3 - Use ide.files.change for live preview
• Step 4 - Use gitlab-smooshpack behind feature flag

What does this MR do?

To fix some issues with Codesandbox Live Preview, we've had to make some changes to the upstream package. While these are pending, we've decided to fork smooshpack with these changes and use this package behind a feature flag:

https://gitlab.com/pslaughter/gitlab-codesandbox-client

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Behind a feature flag, so no changelog
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content