buffer: add constants with max buffer and string lengths
This could be useful for programmers to tell whether a value can be turned into a string or not.
Ref: https://github.com/nodejs/node/issues/13465
/cc @vsemozhetbyt
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
buffer