feat(accordion): add 'show' and 'hide' events

[maxokorokov]

Fixes #4528

[codecov-commenter]

Codecov Report

Patch coverage: 98.09% and project coverage change: +0.44 🎉

Comparison is base (439aa2a) 88.83% compared to head (71f1dcc) 89.27%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4537      +/-   ##
==========================================
+ Coverage   88.83%   89.27%   +0.44%     
==========================================
  Files         115      120       +5     
  Lines        4092     4300     +208     
  Branches      774      820      +46     
==========================================
+ Hits         3635     3839     +204     
- Misses        415      419       +4     
  Partials       42       42              

Flag	Coverage Δ
e2e	50.14% <5.46%> (-2.23%)	⬇️
unit	89.90% <98.05%> (+0.45%)	⬆️

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Impacted Files	Coverage Δ
src/index.ts	100.00% <ø> (ø)
src/scrollspy/scrollspy.ts	94.36% <94.36%> (ø)
src/accordion/accordion.directive.ts	97.32% <100.00%> (+0.23%)	⬆️
src/popover/popover.ts	95.12% <100.00%> (+0.06%)	⬆️
src/scrollspy/scrollspy-config.ts	100.00% <100.00%> (ø)
src/scrollspy/scrollspy.module.ts	100.00% <100.00%> (ø)
src/scrollspy/scrollspy.service.ts	100.00% <100.00%> (ø)
src/scrollspy/scrollspy.utils.ts	100.00% <100.00%> (ø)
src/tooltip/tooltip.ts	98.57% <100.00%> (+0.02%)	⬆️

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