feat: add `preRegisterExternalExtensions` to help external RowDetail

[ghiscoding]

• provide a new preRegisterExternalExtensions() callback in the grid options to help multiple extensions to work together.
• add Row Selection + Row Detail to Example 21
• that will mostly help only Slickgrid-Universal (and won't help Angular/Aurelia projects but it might be useful in the future, who knows)

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.8%. Comparing base (64c4c8b) to head (dbb4ff5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1623 +/- ## ======================================== + Coverage 99.8% 99.8% +0.1% ======================================== Files 198 198 Lines 21796 21805 +9 Branches 7161 7307 +146 ======================================== + Hits 21735 21744 +9 Misses 55 55 Partials 6 6 ```

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