Old-Style ASCII Property Lists

Add Old-Style ASCII Property Lists language support.

Language reference: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/PropertyLists/OldStylePlists/OldStylePLists.html.

Also add support for an extended version of ASCII property list that supports:

• Case insensitive \UXXXX string character encoding
• Array of naturals ([0, 2])
• Custom classes (@myclassname{ myproperty = "value" ; })
• Constants (YES, NO)