fix: normalize backslash in `getStaticPaths()` for windows (#53876)
This normalizes the backslash for windows users who accidentally use `\` in the request paths instead of the expected `/`.
For example, in https://github.com/nodejs/nodejs.org/pull/5617 the slash was hardcoded for split that caused the backlash to remain the the `getStaticPaths()` result.
- Fixes #51225