This PR updates the file icon mappings for JSON (`.json`) file map to
the`json` key. Also, updates `.json` icon from `storage` to `code`.

This allows for the JSON file icons to be replaced in icon themes.

Release Notes:

- Icon themes: Added the ability to change the file icon for JSON
(`.json`) files.

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>