`Move Top Level to File` supporting part files

[FMorschel]

Today if you try to call "Move Top Level to File" on files with parts or part files there is no prompt. I suspect that it has some limitations to the implementation, but I'd like to ask for this to be done after the new enhanced parts release.

I'd also like to point out that similar to this request about asking if the new file should be exported on the original file (so imports don't move) we could have a similar thing (once we can figure out a way of prompting this) about adding new import lines on the part file (if we make one) or keep using the source file imports.

[dart-github-bot]

Summary: The "Move Top Level to File" refactoring currently doesn't handle files with parts or part files. The user requests this functionality to be implemented after the release of enhanced parts, suggesting a similar prompt to the one in issue #56692 for handling imports.