feat: enhance isCError & isWError methods with type predicate

This change allows the methods to act as [type
guards](https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards).

src/lib/cerror.ts

@@ -106,7 +106,7 @@ export class CError extends Error {
 		};
 	}
 
-	public static isCError(obj: unknown): boolean {
+	public static isCError(obj: unknown): obj is CError {
 		return (obj as {[CERROR_SYMBOL]?: boolean})?.[CERROR_SYMBOL] != null;
 	}
 

src/lib/werror.ts

@@ -14,7 +14,7 @@ export class WError extends CError {
 		}
 	}
 
-	public static isWError(obj: unknown): boolean {
+	public static isWError(obj: unknown): obj is WError {
 		return (obj as { [WERROR_SYMBOL]?: boolean })?.[WERROR_SYMBOL] != null;
 	}
 }