Implement Asciidoc Diagnostics in LSP

[vogella]

Implement real-time error reporting for Asciidoc files in the language server.

Details:
The server should parse the document and report issues via the notification.

Key validations to implement:

• Broken Links: Verify that files referenced in include::[] and image::[] directives actually exist.
• Undefined Attributes: Detect usage of document attributes (e.g., {attribute-name}) that have not been defined.
• Syntax Errors: Report malformed structures like tables or lists.