Unwrapped snippet_edit from DOMContentLoaded handler

What does this MR do?

Unwraps SnippetEdit application's handling from DOMContentLoading handler as per #270050 (closed).

This MR also moves some imports as well so that we fetch required modules asap instead of postponing fetching the components with on-demand import.

Screenshots

No visual changes

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #262403 (closed)