Finish basic archive parsing

I finished the basic parsing of archives to retrieve the chat archives. Using the archive library, the archive is loaded, the entries in the Google Chat folder are located, and then they are split into each separate chat.

I struggled a lot trying to figure out the most efficient way to sort the files. Each file has a name that includes the entire path inside, and I needed to extract the files from the folder for each specific space/dm, separate the two data json files from other file data, and wrap the data and files in a MessageGroup object. My problem was that the easiest way to get all the files is to use an enhanced for loop, which makes it hard to sort out the files they way I set up the object. Eventually, I settled on storing the files for each dm or space in a List, which could be looped through later; I want to get the program up and running, and it can be optimised later.

I also spent a lot of time seeing what methods I had, especially on the ArchiveFile class that the extracted files are stored in, and just looking up APIs as I am new to Dart.

At the moment there is not much to see here, as i am doing the basic message parsing before starting the actual gui app.