Fix saving ParseObjects with Pointers as properties

[cbaker6]

Close #168

• Don't skip any key when encoding Pointers
• Add test cases
• Add playgrounds example
• Add entry in changelog
• Prepare for release

[codecov]

Codecov Report

Merging #169 (d2ed16a) into main (c8ae80e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
+ Coverage   82.17%   82.20%   +0.03%     
==========================================
  Files          70       70              
  Lines        6596     6596              
==========================================
+ Hits         5420     5422       +2     
+ Misses       1176     1174       -2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Coding/ParseEncoder.swift	75.16% <100.00%> (+0.32%)	⬆️
Sources/ParseSwift/Types/Pointer.swift	80.00% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	81.52% <0.00%> (-0.16%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	77.54% <0.00%> (+0.26%)	⬆️

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