 transform/urlreplacers: Cache the next position of `urlreplacer.prefix`
 transform/urlreplacers: Cache the next position of `urlreplacer.prefix`

Improved performance due to `bytes.Index` repeated calls

Fixes #5942
gohugoio/hugo,gohugoio/hugo,anthonyfok/hugo,anthonyfok/hugo,gohugoio/hugo,anthonyfok/hugo