Fix source range for NamespacedClass cop

What does this MR do and why?

Fixes: Unable to re-generate all RuboCop TODOs (#366292 - closed)

Related: Work around RubyMine usability/productivity pro... (!89590 - merged)

This reverts Work around RubyMine usability/productivity pro... (!89590 - merged) and replaces it with a better approach by rewriting the cop.

Now, the cop only highlights the class name instead of the whole file in RubyMine.

Thank you @brodock for suggesting this approach!

Screenshots

See that only the class name is underlined, and that the other errors in the file are still visible.

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