Combine AppIconButton and HelpIconButton

[eason9487]

Changes proposed in this Pull Request:

The Button from @wordpress/components supports having an icon. However, the existence of AppIconButton makes it appear that there is a duplication of implementation between them.

Considering that the only use case for AppIconButton in the last four years has been HelpIconButton, this PR combines them into one.

Additional changes:

• Fix the incorrect @return tag in JSDoc
• Remove the forwarding of the rest props because all HelpIconButton use cases only set the eventContext prop.

Detailed test instructions:

1. Go to any page that renders the HelpIconButton component
   • Edit free listings
   • Create campaign
   • Edit campaign
   • Edit store address
   • Onboarding flow
   • Ads-onboarding flow
2. Check if HelpIconButton component is presented the same as before
   
3. Click on the button to see if it can open plugin's document page in a new webpage

Additional details:

💡 In the future, if there are more buttons need to set the position of icon relative to the top of the button text, it's preferred to extend AppButton to support iconPosition="top".

Changelog entry

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.4%. Comparing base (fd83778) to head (e806b2a).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2716   +/-   ##
=======================================
  Coverage     63.4%   63.4%           
=======================================
  Files          339     338    -1     
  Lines         5207    5203    -4     
  Branches      1275    1274    -1     
=======================================
- Hits          3302    3301    -1     
+ Misses        1730    1728    -2     
+ Partials       175     174    -1     

Flag	Coverage Δ
js-unit-tests	63.4% <100.0%> (+<0.1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
js/src/components/help-icon-button/index.js	50.0% <100.0%> (ø)

[ianlin]

Thanks for cleaning up the help icon button components. Confirmed all the help icons displayed correctly, redirected to the right page after clicking it, and the events were tracked accurately. LGTM.