Add TypeScript declaration file

[matt1432]

When used in eslint.config.ts, this plugin is missing types and therefore requires @ts-expect-error.

Similar issue to this: eslint-community/eslint-plugin-promise#488

[ota-meshi]

This project is written in JavaScript and therefore does not have type declarations.

The easiest solution is to submit a PR to https://github.com/DefinitelyTyped/DefinitelyTyped.

If you're interested in rewriting this project in TypeScript, please consider submitting a PR.