Investigate secure coding when archiving with NSKeyedArchiver

[danielsaidi]

When the RichTextDataReader uses NSKeyedArchiver to generate archived data, it doesn't use secure coding. This can lead to errors when the file is unarchived.

For instance, this error was suddenly thrown after editing a file:

UserInfo={NSDebugDescription=value for key 'NS.objects' was of unexpected class 'AFAnalyticsTurnBasedInstrumentationContext' (0x210af22a8) [/System/Library/PrivateFrameworks/AssistantServices.framework].

This seems to be caused by some private frameworks, but we still have to handle it somehow.

We can try to change this RichTextDataReader function:

func richTextArchivedData() throws -> Data {
    try NSKeyedArchiver.archivedData(
        withRootObject: richText,
        requiringSecureCoding: false
    )
}

to use secure coding, but we then have to make sure that the invalid content doesn't cause the archive to fail.