update popup methods addClass / removeClass to return popup instance

[ferdicus]

As mentioned in discussion #3972

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.45%. Comparing base (f4249af) to head (a37eb7b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3975      +/-   ##
==========================================
+ Coverage   86.24%   86.45%   +0.20%     
==========================================
  Files         242      242              
  Lines       32515    32517       +2     
  Branches     2156     2145      -11     
==========================================
+ Hits        28042    28111      +69     
+ Misses       3507     3450      -57     
+ Partials      966      956      -10     

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[HarelM]

Thanks for taking the time to open this PR!
Can you please add a changelog entry and a test to cover the newly added functionality?

[ferdicus]

Thanks for taking the time to open this PR! Can you please add a changelog entry and a test to cover the newly added functionality?

Sorry, have seen this, it's not forgotten :D

Will come back to this during this week 🙇🏿

Sorry for the caused inconvenience

[ferdicus]

@HarelM, would you kindly have another look

thanks 🙇🏿