Make 404 page regex configurable

[lenzls]

We are having the exact same issue as described in #25017.

Scenario

1. I'm requesting /nonsense
2. Our nginx rewrites the URL e.g. to /IT/404.
3. Gatbsy correctly shows the localized 404-page at /nonsense
4. After page load, gatsby changes the location in the browser address bar erroneous to /it/404, while we want it to stay at /nonsense.

Responsible for the navigation is

gatsby/packages/gatsby/cache-dir/production-app.js

Line 148 in 01eeffe

pagePath.match(/^\/(404|500)(\/?|.html)$/) ||

This clause only skips the navigation for pagePaths matching /404, but not for e.g. /IT/404.

Workaround

The suggested solution #25017 (comment) does not work for us. Because for some of our non-404-pages, the navigation is actually desired. Adding catch all rules like /{language}/* and /* will show the 404 page instead of the actual content in these cases. Of course, this breaks our behaviour.

Proposal

I'd like to propose (like #25017 (comment)) to make the 404-page regex configurable, so that we can adapt it to our own 404-page path schema.