Add comments & schema validation to `.files-settings.json`

[nathanshelly]

• schema
  • https://json-schema.org/understanding-json-schema/basics.html
  • https://github.com/neoclide/coc-json#how-to-add-custom-schema-definitionsproperties
• comments
  • https://github.com/stedolan/jq/wiki/FAQ#processing-not-quite-valid-json

[nathanshelly]

Will likely move this to a Nix-based config file