fix(nbButton): allow specifying custom tabindex

[maksim-karatkevich]

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Short description of what this resolves:

Closes #2407

[codecov]

Codecov Report

Merging #2452 into master will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2452      +/-   ##
==========================================
+ Coverage   81.50%   81.52%   +0.01%     
==========================================
  Files         244      244              
  Lines        7382     7387       +5     
  Branches      774      775       +1     
==========================================
+ Hits         6017     6022       +5     
+ Misses       1136     1134       -2     
- Partials      229      231       +2     

Impacted Files	Coverage Δ
...mework/theme/components/button/button.component.ts	84.44% <33.33%> (-2.62%)	⬇️
...ramework/theme/components/toastr/toastr.service.ts	97.39% <0.00%> (+2.60%)	⬆️