[Fix #6902] Naming/RescuedExceptionsVariableName for multiple rescues

[tatsuyafw]

Fixes #6902 .

As issued in #6902, the Naming/RescuedExceptionsVariableName cop did not handle multiple rescues as below.

begin
  # something
rescue FooException => foo # <= flagged
rescue BarException => bar # <= not flagged
end

This PR fixes that and enable the cop handle multiple rescues.

---

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• Added tests.
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[bbatsov]

Thanks!

[tatsuyafw]

Thanks for merging!