This updates redirects' regexes to not match `/_next` paths since this is currently unexpected and can easily cause a multi-match redirect to break loading client-side assets. This also fixes custom-routes not matching correctly when `trailingSlash: true/false` is used
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`
x-ref: https://github.com/vercel/next.js/discussions/24683
x-ref: [slack thread](https://vercel.slack.com/archives/CGU8HUTUH/p1626159845474000)