fix: allow optional param in middle of route

[danieldiekmeier]

I have a route with multiple optional params, but both have matchers that are exclusive to each other. That means for a given URL, there is no ambiguity which param matches.

Consider this example:

Route:           /[user]/[[type=type]]/[[page=page]]
Matcher `type`:  (param) => ["overview", "finished", "pending"].includes(param)
Matcher `page`:  (param) => /^\d+$/.test(param) && Number(param) > 0

Link:            "/daniel/2"
Expected Params: { user: "daniel", page: "2" }
Actual Result:   No match, SvelteKit raises 404

Looking at the code and the test cases, I think this might have been an accidental oversight, just due to the pretty complex routing algorithm.

This fix now allows the variant where all values have been matched and all params have been used.

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