Allow chained optional parameters

[Rich-Harris]

Closes #7548, alternative to #7549.

This is late night code — it's not exactly self-explanatory, but it works. I think. It might be possible to neaten it up a bit, but not now 😴

It solves the /[[lang=lang]]/[...rest] case, where you want /some-path to result in { rest: 'some-path' }. Currently, that won't match anything, because the [[lang=lang]] param 'eats' the first segment even if it doesn't match, which violates intuitions about how this stuff should work.

The solution is more general than just that case, however — any time an optional parameter with a matcher fails to match, we try it again with the next one by essentially shifting the regex matches rightwards.

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Latest commit: 7c91626

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[Rich-Harris]

Note to self - replace the integration test with unit tests

[dummdidumm]

What about /[[a=b]]/[[c=d]]/e? chained is only true for rest in that case right now

[Rich-Harris]

It works, though in the process of double-checking that I found it gives you a false positive if you have a path like /not-b/d/e. Fixed in 7c91626

[dummdidumm]

Ah woops I misread the chained: true logic - but I guess that was a good thing in this case 😄