Allow %-encoding in route filenames

[Rich-Harris]

Closes #5029. I'm at my least confident when dealing with anything encoding-related, so would appreciate a sanity check on this one

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[Conduitry]

Is this a breaking change? Is this related to requiring all special characters in routes to be URL-encoded in the path names - or is that work separate? (Did it already happen and I missed it?)

[Rich-Harris]

That's separate — this is basically just fixing a bug in how Vite names chunks if percent-encoded characters are involved

[Rich-Harris]

I don't think it's a breaking change

[benmccann]

I think this should work, but we should document it

This would be a breaking change for anyone who has % in their route as they'll now be required to encode it as %25