Redirect Web for Safari
It turns out I didn’t need to make my own extension, because Manabu Nakazawa made one. It’s the free ($4 to unlock all features forever) Safari Extension app Redirect Web for Safari.
Nakazawa’s app lets you match URL patterns and redirect them to other patterns. That’s it. You can set your own patterns (using a simple wildcard system or a more powerful but complicated set of regular expressions) and even sync them with your other devices via iCloud. (The app is available for both macOS and iOS.) (If you don’t use Safari, maybe check out the Redirector plug-in instead.)
Previously:
If you have Jeff Johnson's Stop the Madness extension, a Redirect feature is already included. Check out the Redirect tab in the Stop the Madness options. Works in iOS and macOS.
Also for macOS, Stop the Madness works in Safari, Firefox and Chromium-based browsers.