buffer: move setupBufferJS to internal

Stashing it away in internal/buffer so that it can't be used in
userland, but can still be used in internals.

PR-URL: https://github.com/nodejs/node/pull/16391


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>