[developit/mitt] Fix TypeScript example under strict mode (issue #209)

Job description

Fix mitt issue 209: https://github.com/developit/mitt/issues/209 Update the TypeScript example or declarations so the documented strict mode example typechecks without the Record<EventType, unknown constraint error. Keep the change narrowly scoped and add focused type/example coverage if the repository supports it. Deliverables: upstream PR linked to the issue, changed files summary, and test notes. The PR body or a linked issue comment must mention this OpenJobs listing URL.