Do not overwrite any user own existing replication R script for rjournal_article

[cderv]

This was originally posted in #394 (comment)

We need to watch out to not override any existing R file that would be owned by the user and not generated by us.

Two ideas:

• Provide an opt-out mechanism for the purl-ing.
  Something like

  output: 
    rjournal_article: 
      purl: false

• Use a special comment header # Do not edit as done in other tools so that we know that this is our file and we can overwrite.

[cderv]

@Enchufa2 I have made a PR with the second solution. If you want to try and give your thoughts you are more than welcome. Thank you!

[github-actions]

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