refactor!: Add CDVSettingsDictionary class

[dpogue]

Platforms affected

iOS

Motivation and Context

This will replace the extension category on NSDictionary, which has always felt a little bit hacky and has caused issues in weird circumstances due to requiring special build flags to work properly.

Description

Add a new CDVSettingsDictionary class that is used for accessing preferences.

Yes, subclassing NSDictionary is generally considered a bad idea because the internal implementation details are weird and cause problems. However, in this case we're essentially wrapping a real NSMutableDictionary and proxying everything to it, so this should work the way we'd expect it to.

Testing

Added new unit test providing coverage for all the CDVSettingsDictionary methods.

Added a test file to check Swift behaviour as well, but adding it to the test suite project doesn't work (weird Xcode swift errors). I'd like to refactor the test project setup as well, but I'll deal with that after getting through the main CordovaLib project...

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.31%. Comparing base (98cca7f) to head (a4b4e75).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1458   +/-   ##
=======================================
  Coverage   78.31%   78.31%           
=======================================
  Files          16       16           
  Lines        1826     1826           
=======================================
  Hits         1430     1430           
  Misses        396      396           

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[erisu]

Code LGTM