add better support for certain files + maybe some logic reworks

[KTrain5169]

Hopefully add better support for certain file types

checklist in no particular order:

• maybe fix converter logic since the way it's constructed right now works more for text files
• proper/better JSON support
• better YAML support
• microsoft office file support probably (it can work for CSV and XLSX since there's a library for it but not sure for DOCX)
• .md support? very situational
• keep current file support for .txt files only
• deny use of other types of files since it is very much a bad idea to run them through (or at least warn when it's not a properly supported file type)
• backport this to GUI (dependent on GUI #43) and CMD (not sure how best to do so)
• rework saving logic for CMD/GUI since right now it fits more for CLI instead
• make saving logic not immediately initialise with the conversion-history folder
• make the history text files be folders instead because it just makes more sense (this probably will be done in a separate PR)

this will be a very big PR once finished and so uh it'll stay dry for a little bit probably

[KTrain5169]

I also added flake8 config into this PR because it makes sense but it might be better to add it to main right now instead