fix: Improve Hash.digest(encoding) TypeScript return type

[fredikey]

Summary

This PR improves TypeScript typings in src/hash.ts (no runtime behaviour changes)

1) Fix Hash.digest(encoding) return type

Hash.digest(encoding) is typed as Buffer, but it returns a string when encoding param is provided

createHash(...).update(...).digest('base64') // returns Buffer type, not a string type

This appears to be a typo, cuz similar APIs in this package (Blake3.digest() and Hmac.digest()) return string type when encoding is provided

2) Improve hash() return type precision

Added overloads for hash() function, so return type depends on outputEncoding:

• hash(algorithm, data, outputEncoding) -> string
• hash(algorithm, data) -> Buffer

Before, the return type was always string | Buffer

[boorad]

LGTM 🚀

thanks!