fix: allow removing all wildcard handlers in TypeScript

[xiehuanyi]

What is the purpose of this pull request?

• Bug fix

What changes did you make?

mitt<{ foo: string }>().off('*') removes every wildcard listener at runtime, but TypeScript rejects it because the wildcard overload requires a handler. Make that handler optional, matching the documented behavior and the maintainer's example in #150.

Add strict type checks for an omitted handler and explicit undefined, plus a runtime check that named listeners remain registered. Existing checks that reject the wrong wildcard callback type remain in place.

Validation

The new type calls fail on the original declaration with TS2345/TS2769. After the change, npm run bundle and the full npm test suite pass on Node 18 (20 runtime tests, strict type checks and lint). The repository's existing esm loader is incompatible with the local Node 22 runtime, so the suite was run on Node 18 with compatible Node type declarations. No package changes are included.

Is there anything you'd like reviewers to focus on?

The wildcard overload now permits the same handler omission as the named-event overload.

Does this PR introduce a breaking change?

No; it expands the type declaration to accept an existing runtime operation.

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