doc: add added: info for crypto.timingSafeEqual()

crypto.timingSafeEqual() has been added in v6.6.0 cf. #8304

This commit adds the metadata that will display
"Added in: v6.6.0" and that can later be checked on
https://nodejs.org/api/crypto.html#crypto_crypto_timingsafeequal_a_b

PR-URL: https://github.com/nodejs/node/pull/8796


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>