file_icons: Use separate icon key for JSON files (#24432)

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.

---------

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

assets/icons/file_icons/file_types.json

@@ -101,7 +101,7 @@
     "jpeg": "image",
     "jpg": "image",
     "js": "javascript",
-    "json": "storage",
+    "json": "json",
     "jsonc": "storage",
     "jsx": "react",
     "jxl": "image",