Prevent file overwriting

[cllns]

For our generators, we currently overwrite a path if it already exists. We should fix this so it doesn't happen and show a helpful error message letting them know why.

For some cases (e.g. when we make a parent and a child class), we may want to output (and not overwrite) the parent class but continue on and write the child class.

I'm thinking that we can create Hanami::CLI::Files#create which will raise an error if the path exists and use that everywhere we need to. I guess we won't need an update method, since we use some of the methods on dry-files to update content at a certain place.

# Within Hanami::CLI::Files

      def create(path, *content)
        if exist?(path)
          raise FileAlreadyExistsError.new(path) # this is new
        else
          write(path, *content)
        end
      end