fix: front slashes only get escaped now if they aren't already

[nickytonline]

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Which problem is this pull request solving?

Currently Next.js middleware breaks because the edge bundler modifies the regular expression matchers for URLs that Next.js generates.

List other issues or pull requests related to this problem

fixes #375
fixes https://github.com/netlify/pod-ecosystem-frameworks/issues/414
relates to opennextjs/opennextjs-netlify#1919

Describe the solution you've chosen

I've modified the regex to do a negative lookbehind to ensure we don't escape a front slash if it's already escaped.

Describe alternatives you've considered

N/A but as I mentioned in the issue, do we want the edge bundler modifying regex patterns?

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[eduardoboucas]

LGTM! I left just tiny style nits.

Do you want to move this out of draft?