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.

/cc @Fishrock123 @addaleax since we've previously talked about doing this.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

buffer