View and edit generated `config.json` SMAPI files

[erri120]

A lot of SMAPI mods generate a config.json file that the user can edit to configure a mod.

• [ ] Detect new config.json files after running the game.
• [ ] Show these files in the UI
• [ ] Allow users to edit these files in the UI

These config files are part of the core modding experience for Stardew Valley, and we should make an effort to let users easily deal with them.

Related: https://github.com/Pathoschild/SMAPI/issues/928

[erri120]

Done.