Smart single file diff mode

Problem to solve

Turn on file-by-file diff mode based on the number of files that a user can specify in their preferences.

Intended users

User experience goal

Proposal

In #212849 (closed) we will be automatically switching to file-by-file mode based on the number of files and the number of diff lines. I think we could go 1 step further and make this smarter. Instead of just having X number of files, why not allow users to specify the files they care about? For example, as a frontend engineer I don't really need to review any Ruby files as a backend engineer should review these, I only care about .vue,.js,.haml files. What if we allow the user to specify some form of regex that would by used to filter against? Something like (.vue|.html|.js){10} which would mean only show vue,html,js files and if more than 10 enter file-by-file mode.

To make it easier for non-power users as well we could offer some form of visual editor that would allow users to easily specify the files and numbers they want without having to know or understand how the regex would work.

Further details

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Is this a cross-stage feature?

Links / references

/cc @danielgruesso @pedroms