Add return type annotation to checkDomain function

[Lang-Qiu]

Problem Background

The public function checkDomain in src/index.ts was missing a return type annotation. In TypeScript, explicit type annotations improve code readability, maintainability, and enable better type checking during development.

Changes

• Added the return type : boolean to the checkDomain function, as it returns a boolean value from detector.check(domain).result. This aligns with TypeScript best practices for public API functions.

Verification

The change is purely additive for type safety and does not alter the function's behavior or API. To verify:

1. Compile the TypeScript code to ensure no type errors are introduced.
2. Run existing tests to confirm that functionality remains unchanged.