refactor: Move CDVURLSchemeHandler to private API

[dpogue]

Platforms affected

iOS

Motivation and Context

This is only used by the WKWebView plugin, and there's no way to do anything meaningful with it as public API, so it probably shouldn't be public API.

Description

Moved CDVURLSchemeHandler files out of the public API.

Testing

Existing tests pass.

Checklist

• 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 ✅

Project coverage is 80.22%. Comparing base (e327997) to head (be2963a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1477   +/-   ##
=======================================
  Coverage   80.22%   80.22%           
=======================================
  Files          16       16           
  Lines        1871     1871           
=======================================
  Hits         1501     1501           
  Misses        370      370           

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[erisu]

Tested with the Cordova sample app and an app with all Apache Cordova Plugins loaded..

The test also included setting the custom URL scheme.