Provide better support for language comments

[jsinger67]

Currently comments defined in a grammar using the directives %line_comment and %block_comment are simply skipped.
Since comments are often needed in scenarios like code formatting e.g. in language servers, this limits the usefulness of these directives and even forces the user to include comments in his grammar definition explicitly, which can be very cumbersome.

Therefore we need a way to provide comments to the user's grammar processing at parse time.

[jsinger67]

' parol' v0.22.0 in cunjunction with parol_runtime v0.17.0 provides a minimal solution.
During parse time the user is informed about parsed comments via a new method <UserType>GrammarTrait::on_comment_parsed. If the user needs comments she should provide an own implementation of this method.