Missing support for GNUstep base64 NSData

[Artoria2e5]

Modern GNUstep plists have a more efficient data encoding, and it is the <[ base64data ]> format:

GNUstep source that emits the format:
https://github.com/gnustep/libs-base/blob/753c907938c2a8c4d00cf0fbe01b7e0d020f0064/Source/NSPropertyList.m#L1951-L1956 (gnustep/libs-base@9aa5d4c)

GNUstep source that parses the format:
https://github.com/gnustep/libs-base/blob/753c907938c2a8c4d00cf0fbe01b7e0d020f0064/Source/NSPropertyList.m#L1202-L1277
(gnustep/libs-base@3f2fa9d)

(I wish there is a newer documentation page for the GNUstep format, but I have been unable to locate it.)

[3breadt]

I have added support for parsing this newer format, because it apparently is the currently implemented behavior. But this is not documented on the official GNUStep documentation page. So when writing ASCII property lists, the documented hexadecimal format is used, as before.