Adding Wollok educational language

[faloi]

Two years after the last try (#3764), a new attempt to add Wollok as a language, honoring @pchaigno's and @lildude's suggestions.

Checklist:

• 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: .wlk
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/wollok/multipepita
      • https://github.com/wollok/yaaar
    • Sample license(s): MIT
  • I have included a syntax highlighting grammar: example
  • I have updated the heuristics to distinguish my language from others using the same extension.

[pchaigno]

Among 2656 files downloaded with Harvester, I counted 1132 repositories by 514 users. Usage looks okay to me! The pull request too.