Fix eslint-plugin-json v2+ breaking change: require config in .eslintrc.js

[MatrixNeoKozak]

The eslint-plugin-json v2+ changed how it is configured. Previously it automatically applied its rules when added as a plugin, but now you must explicitly set the 'json/*' rules or extend 'plugin:json/recommended'. Without this, the plugin does nothing, and JSON files are not linted. The fix adds extends: ['plugin:json/recommended'] to the root .eslintrc.js (where JSON linting is intended) and adds the 'json' plugin in the correct place.

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