Implement Asciidoc Document Outline (LSP)

[vogella]

Implement the textDocument/documentSymbol request in the language server to provide a hierarchical view of the document structure.

Details:

• Parse the Asciidoc document to identify section headers (=, ==, ===, etc.).
• Return a tree of DocumentSymbol objects representing the document's structure.
• This will enable the 'Outline' view in the IDE, allowing users to quickly navigate large documents.