[TIMOB-18062] iOS: AttributedString Parity with Android

Jira: https://jira.appcelerator.org/browse/TIMOB-18062

iOS deprecation of Titanium.UI.IOS.AttributedString in favor of Titanium.UI.AttributedString. Pull request include Android support for AttributedString from pull request https://github.com/appcelerator/titanium_mobile/pull/6358, which will be closed once this PR is reviewed and merged.

Includes fix for font property in AttributedString, where fontsize was not working.
attributedString property will be implemented in TextArea and TextField on new ticket. This ticket will just address android new attributedString feature for Label, and parity between android and iOS.