Add experimental caseSensitiveRoutes config (#50869)
This adds an experimental `caseSensitiveRoutes` config that currently applies for `rewrites`, `redirects`, and `headers` to change the default of case-insensitive.
x-ref: [slack thread](https://vercel.slack.com/archives/C02K2HCH5V4/p1686080359514479?thread_ts=1686077053.623389&cid=C02K2HCH5V4)
x-ref: [slack thread](https://vercel.slack.com/archives/C057RG6Q9MX/p1686078875948069?thread_ts=1686077882.133609&cid=C057RG6Q9MX)
x-ref: https://github.com/vercel/next.js/issues/21498
