expose a function for curve25519-js

[zorostang]

I think it would make sense to expose a signing (and verification) function for consumers of secretjs as a function to sign and verify any arbitrary piece of data with a specified key.

This would eliminate a dependency from new frontend projects and would help with onboarding.

Here's a new CCR that uses the depency but it would be great if secretjs could have been leveraged for that need
https://github.com/zorostang/nft-authorization-front-end/blob/a50441fdf2c887dcb53319f34f48190e635534ec/src/main.ts#L238