feat(dx): warn when addRoute cannot find the parent

[skirtles-code]

This adds a warning when calling router.addRoute('foo', { ... }) if the parent route isn't found.

Without this warning, it can be difficult to tell why addRoute isn't working correctly. With an absolute path, beginning with /, the addition seems to succeed, but the new route won't be a child of the intended parent. With a relative path, it'll fail to add the new route, but with a confusing error about the path being invalid.

I've also added a test for the similar warning in removeRoute, though the warning itself already existed.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.90%. Comparing base (13303bd) to head (364cd04).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2157   +/-   ##
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  Coverage   90.90%   90.90%           
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  Files          24       24           
  Lines        1121     1121           
  Branches      347      347           
=======================================
  Hits         1019     1019           
  Misses         63       63           
  Partials       39       39           

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[posva]

Thanks!