mina-signer package is using literal bigint, fail to build with react-native

[chezhe]

react-native doesn't support literal bigint like:

const a = 2n

Hope the syntax can be

const a = BigInt(2)

I've met same problem with aptos(aptos-labs/aptos-core#3943), they changed it to above.

I think it can be achieved by configuring babel.

I didn't find the branch of release 1.7.0, I don't know how to submit a PR.