Add routeFilePrefixPattern to the API of file based routing

[ExcellentAmericanEagle]

Currently, the API of file based routing provides the option routeFilePrefix to specify the prefix all files must have to be considered for routing.

The opposite - ignoring certain file paths from being treated as routing files features two options:

• routeFileIgnorePrefix which accepts a plain string and
• routeFileIgnorePattern which accepts a regex string and thus much more powerful.

It'd be helpful to have a pattern option equivalent to routeFilePrefix as well. It'll make the most sense to name it routeFilePrefixPattern - and it'd also take a regex string to be as similar to existing routeFileIgnorePattern as possible.

Additional context - possible use case

In our project we have the convention that index.tsx are the only way to define routes in the project. All other file names shouldn't be treated as routes. My team finds it much more cleaner and familiar to Next.js's App Router whereas page.tsx is used to define roues.

In theory we can just set set routeFilePrefix to index.tsx - but it won't work. That's because there's the special file __root.tsx that must not be ignored by the generator. Defining routeFilePrefix to index.tsx would essentially make anything but index.tsx ignored. That's why we came up with the idea of having an option like routeFilePrefixPattern.

Alternative solution 1

Perhaps routeFilePrefix could also accept an array of strings (string[]) to allow matching multiple file prefixes.

So, we wouldn't need need routeFilePrefixPattern and routeFilePrefix: ["index.tsx", "__root.tsx"] would be sufficient

Alternative solution 2

Because __root.tsx is a mandatory file, why would one want to make the generator ignore this file? This name is hardcoded and not changeable.