Add `buffer.constants`, containing length limits for `Buffer` and
`string` instances.
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
PR-URL: https://github.com/nodejs/node/pull/13467
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>