def within_files(file_pattern, options={}, &block)
      return if @sandbox

      if (!@ruby_version || @ruby_version.match?) &&
        (!@gem_spec || @gem_spec.match?)
        Rewriter::Instance.new(self, file_pattern, options, &block).process
      end
    end
def within_files(file_pattern, options={}, &block)
      return if @sandbox

      if (!@ruby_version || @ruby_version.match?) &&
        (!@gem_spec || @gem_spec.match?)
        Rewriter::Instance.new(self, file_pattern, options, &block).process
      end
    end
Parse within_files dsl, it finds specified files.
 It creates a [Synvert::Core::Rewriter::Instance] to rewrite code.

 @param file_pattern [String] pattern to find files, e.g. spec/**/*_spec.rb
 @param options [Hash] instance options.
 @param block [Block] the block to rewrite code in the matching files.