Exclude *.gemspec by default

[coding-red-panda]

Is your feature request related to a problem? Please describe.
When using this gem inside a Gem project, all text lines in the Gemspec file will be marked as missing a decorator.

Describe the solution you'd like
ANY gemspec file to be ignored by default

Describe alternatives you've considered
Manually adding a line to RuboCop's config file for every cop, or explicitly adding gemspec files to Global exclude sections

Additional context
Gemspecs should be as lean as possible, and I feel it better to have the Cops ignore these gemspecs by default, and not having to extend my configuration either globally for large projects/custom configurations or to list this lien for every Cop