refactor: Modernize some ObjC code

[dpogue]

Platforms affected

ios

Motivation and Context

Some of the code in CordovaLib dates back to the very early dates of cordova-ios and an era when iOS 4 introduced breaking changes and Objective C was having new language features introduced (yes, Cordova actually predates the transition to ObjC ARC 💀).

Nowadays there are some conventions that are pretty standard (like instancetype for initializer methods) that we should aim to adopt for consistency.

Also, refactored CDVConfigParser to avoid needing a round-trip through weird CDVViewController methods to do anything useful. I can split this commit out into its own PR if that makes it easier for review.

Description

• Various small ObjC cleanups
• Add some static methods to CDVConfigParser to facilitate parsing config.xml

Testing

All existing unit tests pass.

Checklist

• [x] I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.22%. Comparing base (f3339a9) to head (26872aa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1472 +/- ## ======================================= Coverage 80.22% 80.22% ======================================= Files 16 16 Lines 1871 1871 ======================================= Hits 1501 1501 Misses 370 370 ```

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