Rename Perl 6 to Raku

[KamilaBorowska]

Description

Perl 6 has changed its name to Raku.

This PR introduces new extensions that aren't really used much (most code is using .p6 or .pm6) considering the support for those was introduced a week ago. However, according to https://github.com/perl6/problem-solving/blob/master/solutions/language/Path-to-Raku.md, .raku, .rakutest and .rakumod are the new extensions, and support for those is already in main Raku implementation (see changelog https://github.com/rakudo/rakudo/releases/tag/2019.11, although worth noting this only adds .rakumod as other extensions don't require implementation change). I don't think introducing those extensions will be an issue as they unambiguously refer to Raku and shouldn't conflict with anything else.

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=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am adding a new language.

  • The extension of the new language 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=extension%3AFOOBAR+KEYWORDS+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a syntax highlighting grammar: https://github-lightshow.herokuapp.com/
  • I have updated the heuristics to distinguish my language from others using the same extension.

• I am fixing a misclassified language

  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • [URL to each sample source, if applicable]
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

• I am changing the source of a syntax highlighting grammar

  • Old: https://github-lightshow.herokuapp.com/
  • New: https://github-lightshow.herokuapp.com/

• I am updating a grammar submodule

• I am adding new or changing current functionality

  • I have added or updated the tests for the new or changed functionality.

• I am changing the color associated with a language

  • I have obtained agreement from the wider language community on this color change.
    • [URL to public discussion]
    • [Optional: URL to official branding guidelines for the language]

[KamilaBorowska]

I think this can be merged without adding extensions if that's the only blocker (I already removed them)?

[Alhadis]

@xfix We don't merge unless a GitHub staff member has given their approval; that's just policy. 😉 In any case, changes won't take effect across the site until the next release of Linguist has been made, which generally happens roughly once a month.

[KamilaBorowska]

Fair enough, I wasn't sure with "Pending Popularity" label.

[pchaigno]

Sorry, I should have explained. I'm adding that label to remember to collect usage for these extensions. This can be merged as soon as @lildude gives his 👍