[5.1] fix: Do no sort listeners differently depending on their type

[phil-davis]

With array_multisort, when priority is the same it would fallback to
comparing values, and PHP will sort Closures after array callables.
This is not something we want as it means turning an array callable into
a Closure would potentially change its order, leading to unexpected
side-effects.

Backport #162 to 5.1 branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.45%. Comparing base (930e42f) to head (5cd7d4d).
⚠️ Report is 4 commits behind head on 5.1.

Additional details and impacted files

@@             Coverage Diff              @@
##                5.1     #169      +/-   ##
============================================
- Coverage     99.45%   99.45%   -0.01%     
  Complexity      119      119              
============================================
  Files             7        7              
  Lines           367      366       -1     
============================================
- Hits            365      364       -1     
  Misses            2        2              

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