[TIMOB-18062] iOS: AttributedString Parity with Android

Jira: https://jira.appcelerator.org/browse/TIMOB-18062

This PR is a continuation of https://github.com/appcelerator/titanium_mobile/pull/6409/ which came from #6358.

It addresses the comments by @pingwang2011 for the Android portion of the code. The doc is still not completed yet as Titanium.UI.iOS.Attribute and Titanium.UI.iOS.AttributedString yml files are missing the "deprecated".

The propertyAccessors is now used in AttributedStringProxy.java, AttributeProxy.java and LabelProxy.java.
Why are these constants at the TiProxy layer. These should be in the `UIModule`. I can now do something like this on iOS

`var win = Ti.UI.createWindow();Ti.API.info(win.ATTRIBUTE_FONT);`