[2.x] Create new navigation destination class to replace external routes

[emmadesilva]

Targets #1568, instead of #1635

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 99.87%. Comparing base (9353eef) to head (ba65f1c).

❗ Current head ba65f1c differs from pull request most recent head 976d7ff. Consider uploading reports for the commit 976d7ff to get more accurate results

Additional details and impacted files

@@                         Coverage Diff                         @@
##             improved-navigation-internals    #1636      +/-   ##
===================================================================
+ Coverage                            99.85%   99.87%   +0.02%     
- Complexity                            1783     1787       +4     
===================================================================
  Files                                  185      185              
  Lines                                 4812     4816       +4     
===================================================================
+ Hits                                  4805     4810       +5     
+ Misses                                   7        6       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.