feat(typeahead): allow custom class on window for modal issue

[eliprand]

This PR enables providing one or more custom classes to the typeahead window. It's intended to help solve #3778 as per my comment.

I modeled the behavior and implementation after windowClass from NgbModal.The main intent here is allowing users to provide a custom z-index value along with the container="body" option for their specific needs. But ultimately, it can be used to override any attributes on the ngb-typeahead-window.

I did not feel we needed to update the demo, but I would be happy to do so if needed.

Before submitting a pull request, please make sure you have at least performed the following:

• read and followed the CONTRIBUTING.md and DEVELOPER.md guide.
• built and tested the changes locally.
• added/updated any applicable tests.
• added/updated any applicable API documentation.
• added/updated any applicable demos.

[codecov-io]

Codecov Report

Merging #3947 (8df7971) into master (dce3886) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3947   +/-   ##
=======================================
  Coverage   90.70%   90.70%           
=======================================
  Files         110      110           
  Lines        3185     3186    +1     
  Branches      609      609           
=======================================
+ Hits         2889     2890    +1     
  Misses        204      204           
  Partials       92       92           

Flag	Coverage Δ
e2e	49.23% <100.00%> (+0.01%)	⬆️
unit	87.41% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/typeahead/typeahead-window.ts	100.00% <ø> (ø)
src/typeahead/typeahead.ts	96.03% <100.00%> (+0.03%)	⬆️

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[maxokorokov]

LGTM, thanks!

[eliprand]

@maxokorokov thanks for merging. FYI, can I/we update the inline docs for the typeahead.ts changes I made? I was optimistic and say @since 8.1.0 but it looks like we're going out with 9.1.0. Can I update this code here? or is it too late?

[maxokorokov]

@eliprand we're usually doing it before the feature release, don't worry
I've also renamed windowClass to popupClass later, because publicly (in docs) everywhere we're referring to it as typeahead popup