Simplified usage for type guards

I'm currently using this package with a helper to create type guards:
```ts
/** Create a type guard function for a class decorated with validators. */
export function isValidFactory(classType: ClassType): (instance: object) => instance is T {
return (instance: object): instance is T => {
try {
transformAndValidateSync(classType, instance)
return true
} catch (err) {
return false
}
}
}
```

Would you be interested in a PR that adds this helper?

Start by locating the existing transformAndValidateSync helper, ClassType definition, and public exports. If the proposal is accepted, add the type-guard helper and verify behavior for valid and invalid instances using the package's existing validation tests.

Written by the indexing model from the issue text.