refactor: type npm-login result (#104)

Add type for the return of `npmLogin`.

src/cmd-login.ts

@@ -83,6 +83,11 @@ export const login = async function (
   return 0;
 };
 
+/**
+ * The result of a login attempt. Either success with the token, or failure.
+ */
+type LoginResult = { code: 0; token: string } | { code: 1 };
+
 /**
  * Return npm login token
  */
@@ -91,7 +96,7 @@ const npmLogin = async function (
   password: string,
   email: string,
   registry: RegistryUrl
-) {
+): Promise<LoginResult> {
   const client = getNpmClient();
   try {
     const data = await client.adduser(registry, {