feat(Contract): add TS support

Author: bigslycat

src/index.d.ts

@@ -28,3 +28,26 @@ export declare class UnionError extends ValidationError {
   );
 }
 
+export type Contract<T> = {
+  (valueName: string): {
+    (value: unknown): ValidationError | T,
+    optional(value: unknown): ValidationError | void | T,
+    maybe(value: unknown): ValidationError | void | null | T,
+  },
+  (valueName: string, value: unknown): ValidationError | T,
+
+  optional(
+    valueName: string
+  ): (value: unknown) => ValidationError | void | T,
+  optional(valueName: string, value: unknown): ValidationError | void | T,
+
+  maybe(
+    valueName: string
+  ): (value: unknown) => ValidationError | void | null | T,
+  maybe(valueName: string, value: unknown): ValidationError | void | null | T,
+};
+
+export declare function of<T>(
+  validate: (valueName: string, value: unknown) => ValidationError | T,
+): Contract<T>;
+