Introduce invoke_callback helper method in WP_Ability class

[gziolo]

It's a work in progress — an attempt to improve handling around callback in the WP_Ability class. In particular, I want to explore whether it would be valuable to bind anonymous functions to the ability instance to offer access to its methods.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.86%. Comparing base (4beab25) to head (e0d09cb).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk      #88      +/-   ##
============================================
+ Coverage     84.84%   84.86%   +0.01%     
  Complexity      102      102              
============================================
  Files            16       16              
  Lines           772      773       +1     
  Branches         86       86              
============================================
+ Hits            655      656       +1     
  Misses          117      117              

Flag	Coverage Δ
javascript	92.66% <ø> (ø)
unit	81.80% <100.00%> (+0.03%)	⬆️

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