[2.x] Experiment with splitting out the route model

[emmadesilva]

Targets #1568 and experiments with extracting a PageRoute model.

This may add too much complexity, and not solve the right problem (as the issue is with ExternalRoutes being hacky, but since they are new and just used in navigation it may be better to improve that instead), but I wanted to visualize what this could look like before making a decision.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.73%. Comparing base (a2baea5) to head (510649a).

❗ Current head 510649a differs from pull request most recent head d5fa496. Consider uploading reports for the commit d5fa496 to get more accurate results

Additional details and impacted files

@@                         Coverage Diff                         @@
##             improved-navigation-internals    #1635      +/-   ##
===================================================================
- Coverage                            99.85%   99.73%   -0.13%     
- Complexity                            1783     1786       +3     
===================================================================
  Files                                  185      186       +1     
  Lines                                 4812     4818       +6     
===================================================================
  Hits                                  4805     4805              
- Misses                                   7       13       +6     

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[emmadesilva]

Yeah, no. This is definitely not solving the right problem.