Add syntax highlighting for Crystal programming language

What does this MR do?

Highlight Crystal code using a lexer plugin rouge-lexers-crystal.

Until this PR https://github.com/jneen/rouge/pull/441 is merged we want to use this plugin to make Crystal users happier :)

Are there points in the code the reviewer needs to double check?

Please double-check that Crystal code is highlighted correctly.

Why was this MR needed?

Before this commit Crystal code on GitLab was not highlighted. After this commit all Crystal code has colors! 🎉

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conform to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• https://github.com/jneen/rouge/pull/441 (pending)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/35409