fix: wrong types definition specified for class field

According to [official docs for customer creation](https://razorpay.com/docs/api/customers/create/) using razorpay API, `fail_existing` should be a string.

But the types defined where `boolean`, numeric `1` & `0`.

fixes razorpay#377, fixes razorpay#381

lib/types/customers.d.ts

@@ -25,7 +25,7 @@ export declare namespace Customers {
          *
          * `1` (default): If a customer with the same details already exists, throws an error.
          */
-        fail_existing?: boolean | 0 | 1;
+        fail_existing?: "0" | "1";
         /**
          * Customer's GST number, if available
          */
@@ -245,4 +245,4 @@ declare function customers(api: any): {
      fetchEligibility(eligibilityId: string): Promise<Partial<Customers.RazorpayCustomerEligibility>>
 }
 
-export default customers
+export default customers