feat: make "current" ParseObjects immutable

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Currently all objects can be set through current, like: ParseUser.current, ParseInstallation.current, ParseConfig.current. These should be mutable and when the developer wants to mutate, they make mutable copies. This prevents setting to nil directly as current should only happen when using the logout method.

Related issue: #267

Approach

Make all current properties internally set to the SDK.

All playgrounds examples have shown how to make mutable copies instead of setting current directly for some time now. Many people learn how to use the SDK from the Swift playground examples.

TODOs before merging

• Make sure all test cases pass
• Verify everything works in Swift Playgrounds
• Improve/remove force unwrapping in ParseACL
• Improved comments in SDK
• Add entry to changelog

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #266 (91ab18a) into main (5860abd) will decrease coverage by 0.00%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #266      +/-   ##
==========================================
- Coverage   82.33%   82.32%   -0.01%     
==========================================
  Files          99       99              
  Lines       10512    10512              
==========================================
- Hits         8655     8654       -1     
- Misses       1857     1858       +1     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseInstallation.swift	77.36% <ø> (ø)
Sources/ParseSwift/Types/ParseConfig.swift	97.05% <ø> (ø)
Sources/ParseSwift/Types/ParseACL.swift	86.30% <87.50%> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	79.85% <100.00%> (+0.21%)	⬆️
Sources/ParseSwift/Objects/ParseObject.swift	74.57% <0.00%> (-0.57%)	⬇️

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