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
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
buffer