fix(eventEmitter): improve event handling and listener management

[yslpn]

Description

1. I expect that the same function will not be added twice. To fix this, I switched from arrays to Set.
2. Ensures that the event invocation cycle will not be broken if the handlers themselves start changing the list of listeners. For example, adding or removing.

Types of changes

Refactored EventEmitter + add two new tests

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Examples update

Checklist

• I have read the CONTRIBUTING and CODE_OF_CONDUCT docs
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.81%. Comparing base (6bb8983) to head (3991251).
Report is 198 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2277      +/-   ##
==========================================
- Coverage   77.05%   76.81%   -0.24%     
==========================================
  Files          84       89       +5     
  Lines        2157     2497     +340     
  Branches      555      648      +93     
==========================================
+ Hits         1662     1918     +256     
- Misses        382      463      +81     
- Partials      113      116       +3     

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[vonovak]

Hello and thanks for the PR!

Firstly, may I ask what's the motivation for the PR?

I'd say it can be merged, but for v6 - the change of behavior, while subtle, is a breaking change. Also the removal of removeListener is a breaking change - it should be added back.

Thank you :)