[2.x] Create positioning priority constants in the navigation menu class

[caendesilva]

Extracts constants for the default priorities used for navigation menu design.

NavigationMenu::DEFAULT = 500
NavigationMenu::LAST = 999

For example, the following is not very clear: (What does 999 mean and why do we use it?

$priority ?? 999

But here, with the constant it's clear that we are falling back to putting the item last in the menu.

$priority ?? NavigationMenu::LAST

It also makes things like this clearer

$priority = NavigationMenu::LAST + 1

And a good IDE also shows the underlying integer values of the constants.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (cb580ed) to head (aaa17a0).

❗ Current head aaa17a0 differs from pull request most recent head 59175a3. Consider uploading reports for the commit 59175a3 to get more accurate results

Additional details and impacted files

@@                       Coverage Diff                        @@
##             improved-navigation-internals    #1598   +/-   ##
================================================================
  Coverage                            99.95%   99.95%           
  Complexity                            1776     1776           
================================================================
  Files                                  183      183           
  Lines                                 4800     4800           
================================================================
  Hits                                  4798     4798           
  Misses                                   2        2           

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