Resolves #7716.

Currently if we have a config like Config.string("greeting") and a config provider like ConfigProvider.fromMap("greeting" -> "Hello, World!") the result of loading this config will be Hello.

While this has some internal logic to it since the , is a sequence delimiter and if a sequence isn't requested the first value will be returned, this also seems somewhat unexpected to me. Not only is the additional information in the value silently dropped but I also said I wanted to just load a string and not a sequence of strings.

This PR changes that behavior so that now we only split a string if a sequence is requested. So loading Config.chunkOf(Config.string("greeting")) would return Chunk("Hello", "World!") whereas loading Config.string("greeting") would just load "Hello, World!".

Unfortunately, this does require significant contortions in the implementation of Flat to support this while preserving binary compatibility since the current signature of Flat#load does not give the implementation enough information to know whether it is being requested to load a single value or a collection of values.