Cop/StaticTranslationDefinition: Allow constant assignments for Structs

What does this MR do and why?

Prior this commit, 👮 Cop/StaticTranslationDefinition would flag the following code:

SomeClass = Struct.new do
  def text
     _('Some translated text')
  end
end

This commit allow such style again so workarounds are not longer needed like:

Struct.new('SomeClass') do
  def text
    _('Some translated text')
  end
end

Closes #338295 (closed)

How to set up and validate locally

Run the specs 😅

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