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

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

Affected core subsystem(s)

buffer