[2.x] Simplify the navigation items class

[emmadesilva]

Targets #1568 and further simplifies the navigation internals. It inlines the added navigation destination class and simplifies the navigation item class so it is now at the point where I can't find anything else to remove, signaling it has reached perfection.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (3eca812) to head (09938c8).

❗ Current head 09938c8 differs from pull request most recent head 3e4901b. Consider uploading reports for the commit 3e4901b to get more accurate results

Additional details and impacted files

@@                         Coverage Diff                         @@
##             improved-navigation-internals    #1637      +/-   ##
===================================================================
+ Coverage                            99.87%   99.93%   +0.06%     
+ Complexity                            1787     1779       -8     
===================================================================
  Files                                  185      184       -1     
  Lines                                 4816     4802      -14     
===================================================================
- Hits                                  4810     4799      -11     
+ Misses                                   6        3       -3     

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