Check modpack directory for manifest file

Check whether any of

• manifest.json
• minecraftinstance.json
• config.json
• mmc-pack.json
• instance.cfg
• modrinth.index.json

exist. If they do

• ask the user whether they want values from these to be loaded.
• display comparison of values which will be loaded to current settings
• allow selection of values to load
• offer to load all
• offer to refuse