feat: server js fallthrough to page if handler doesn't exist

[gtm-nayan]

closes #10102

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[dummdidumm]

Needs to merge master in, master contains some changes which adjusts response after the if/else chain which would no longer apply.

[benmccann]

did you mean for this to be 405? based on the comment I would have expected 404

[teemingc]

Yeah, I think it's 405. Kit responds with 405 method not allowed when the method handler isn't found.

[benmccann]

@gtm-nayan it looks like this PR still needs a rebase

[dummdidumm]

Through #11256 the underlying issue of conflicting routes was fixed, and as such this PR is no longer necessary - closing.