Switch native component to optionalDependency

[junderw]

Current:

• installing this without native compiler will fail, any app using this as a dependency can not npm install.

Expected:

• installing this without native compiler will fall back on JS implementation.

Separated the component here: https://github.com/bitcoinjs/tiny-secp256k1-native

This should allow people to properly fall back on the JS implementation.

[junderw]

Thanks for the review.