Fix TypeScript typing of bigints (#12)

Author: DASPRiD

index.d.ts

@@ -40,7 +40,7 @@ declare const fnv1a: {
 	//=> 13487074350300261116944693128525960095n
 	```
 	*/
-	bigInt(string: string, options?: fnv1a.Options): BigInt;
+	bigInt(string: string, options?: fnv1a.Options): bigint;
 };
 
 export = fnv1a;

index.test-d.ts

@@ -2,4 +2,4 @@ import {expectType} from 'tsd';
 import fnv1a = require('.');
 
 expectType<number>(fnv1a('🦄🌈'));
-expectType<BigInt>(fnv1a.bigInt('🦄🌈', {size: 128}));
+expectType<bigint>(fnv1a.bigInt('🦄🌈', {size: 128}));