src: change macro to fn

Change base64_encoded_size and unbase64 to inline functions. The
base64_encoded_size is a constexpr to be used in function declarations.

PR-URL: https://github.com/nodejs/node/pull/23603


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>