Implement Context-Aware Content Assist for Asciidoc (LSP)

[vogella]

Replace the current static completion list with a context-aware implementation in the language server.

Details:

• Attributes: Suggest defined document attributes when typing :.
• Headers: Suggest appropriate header levels based on the current nesting depth.
• Blocks: Suggest block types (e.g., [source], [note]) only where appropriate.
• Images/Includes: Improve the existing file-system scanning to be more robust and context-sensitive.