Enable completion item resolution so selecting a completion can populate
documentation with a preview of the corresponding .def declaration.
- Turn on resolve_provider in the completion options.
- Implement on_completion_resolve to attach Markdown documentation
computed by def_hover_markdown when the completion item's data
contains a ("def" -> [kind, name]) payload.
- Include that "def" payload when generating def-related completion items.
- Add tests that verify block-template and address-list items resolve to
Markdown previews.
Before: def-related completion items had no documentation on resolve.
After: selecting such items will return a Markdown preview of the .def entry.
Christoph
17fb79a346