Support scanning .gemspec files

[reedloden]

I noticed that GitHub's Dependabot checks .gemspec files, as well as Gemfile.lock. We should do the same, as it would let bundler-audit monitor gems directly.

They also check other files as well, so may also want to add that.

See https://github.com/dependabot/dependabot-core/tree/main/bundler/lib/dependabot/bundler for how they do it.

[postmodern]

I believe bundler already includes any dependencies from the .gemspec into Gemfile.lock, if the Gemfile contains gemspec.Also .gemspec does not contain the exact gem versions, but only the version ranges, so we'd need additional logic for determining whether a version range intersects with a vulnerable version range.

[postmodern]

It might be useful for indicating to the user that they need to change the version range in the .gemspec vs just running bundle update, but that would require some clever to logic to detect overly strict version ranges in the .gemspec vs. the Gemfile.lock.