Implement "spark tables" functionality, which allows users to roll
dice against markdown tables to retrieve specific values.
- Add `sparkTablesSource` to scan markdown files for tables starting
with a dice notation (e.g., d6, d20).
- Implement `/spark` command in the journal to resolve and roll
spark tables.
- Add a new message type `spark` with a dedicated UI component to
render the results.
- Update completions API to include spark table metadata.
