CommentSyntax setting should be path based

[pikinier20]

The -comment-syntax setting tells Scaladoc whether the docstrings are written in Markdown or Wiki syntax. For now, the comment syntax setting can only be set globally for entire project. In Dotty we have the Scala 2.13 standard library which uses Wiki syntax and Scala 3 standard library which uses Markdown.

Currently we use the Wiki syntax for entire project because Wiki parser handles Markdown quite well.

Having in mind that new comments should be written in Markdown syntax, there are few features that work only with Markdown (e.g. snippet compiler) and therefore it would be nice if we could set correct syntaxes for specific libraries in Dotty.

Also, it might help in migration process of other libraries from Scala 2 to Scala 3.

The implementation should be easy because there's PathBased abstraction that is already used for source links and snippet compiler. Using this abstraction we can decide which syntax should be used based on the path to the class file.