feat: introduce devMode to support nodejs based unit testing

[ghiscoding]

• copies implementation made in 6pac/SlickGrid repo at feat: introduce devMode to support nodejs based unit testing 6pac/SlickGrid#946

as the title suggests this is meant to make unit/integration tests, e.g with testing-library, possible for apps using Slickgrid. In my specific case I'm making use of angular-slickgrid and would like to be able to test my app using angular-testing-library.

The introduced devMode workarounds are in order to work around the following jsdom issues:

jsdom/jsdom#2310 (missing clientWidth) jsdom/jsdom#992 (missing ownerNode, even with a nice reference to Slickgrid ;) ) For additional conversations about this topic please refer to ghiscoding/Angular-Slickgrid#1319

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (102e299) 86.53% compared to head (3fde9b2) 86.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1251      +/-   ##
==========================================
+ Coverage   86.53%   86.54%   +0.01%     
==========================================
  Files         196      196              
  Lines       21380    21381       +1     
  Branches     7098     7099       +1     
==========================================
+ Hits        18500    18501       +1     
  Misses       2880     2880              

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