Add new Rails/JSONSymbolizeNames cop

[viralpraxis]

I've seen this pattern a lot:

JSON.parse(large_json).deep_symbolize_keys

instead of travesing Ruby hash once more we can use the symbolize_names option:

JSON.parse(large_json, symbolize_names: true)

Caveats / FP scenarios:

1. symbolize_names does not work if create_addition option is provided.
2. User might use both symbolize_names: false and deep_symbolize_keys.
3. There's no autocorrection yet, but it should be relatively easy to add (at least for some cases)

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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.