Add support for annotations

[artworx]

The following code is not highlighted properly:

class Aaaa
  annotation Moo
  end
end

See crystal-lang/crystal#6063

[rhysd]

Could you provide the documentation of annotation? I want to know the detail of the macro before trying to support it.

[artworx]

The documentation is a work in progress, see crystal-lang/crystal-book#292

The only documentation available is the pull request description I mentioned in the initial issue.