Handle to_h with block in Rails/IndexBy and Rails/IndexWith

[eugeneius]

Followup to #208.
See also rubocop/rubocop#8517.

Since Ruby 2.6, Enumerable#to_h accepts a block which is called with each element and must return a key and value to include in the new hash: https://rubyreferences.github.io/rubychanges/2.6.html#to_h-with-a-block

If the element is always returned as either the key or the value, using Enumerable#index_by or Enumerable#index_with is faster and clearer.

This is similar to the existing logic that detects map { ... }.to_h.

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[koic]

Thanks!