Fix TypeScript typing of bigints

[DASPRiD]

The typescript typehints were incorrect. They were refering to the BigInt class instead of the bigint type. This made typescript trip in this situation:

const foo = 1n;
const bar = foo - fnv1a.bigInt('foobar');

In this case, typescript compiler would complain that the minus operator does not work with bigint and BigInt. Workaround without this patch:

const bar = foo - (fnv1a.bigInt('foobar') as bigint);