Add {.,}latexmkrc to Perl

[ObserverOfTime]

Description

Latexmk uses Perl scripts as configuration files.
This pull request makes it so they are recognised as such.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=filename%3Alatexmkrc+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): https://mg.readthedocs.io/latexmk.html#configuration-files
    • Sample license(s): N/A
  • I have included a change to the heuristics to distinguish my language from others using the same extension.