Fix #1068 Throw an error if both socketMode: boolean and receiver: Receiver arguments in App constructor

[seratch]

Summary

This pull request fixes #1068 . I'm feeling that the error message can be improved. Suggestions would be appreciated!

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1077 (3d846f4) into main (02d601d) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1077      +/-   ##
==========================================
+ Coverage   68.73%   68.87%   +0.13%     
==========================================
  Files          13       13              
  Lines        1206     1208       +2     
  Branches      355      356       +1     
==========================================
+ Hits          829      832       +3     
  Misses        304      304              
+ Partials       73       72       -1     

Impacted Files	Coverage Δ
src/App.ts	83.19% <100.00%> (+0.37%)	⬆️

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[seratch]

I think this one is ready for merge but just in case, I'll wait for others' comments until tomorrow.

[srajiang]

Looks good.