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>

doc/api/crypto.md

@@ -1418,6 +1418,9 @@ keys:
 All paddings are defined in `crypto.constants`.
 
 ### crypto.timingSafeEqual(a, b)
+<!-- YAML
+added: v6.6.0
+-->
 
 Returns true if `a` is equal to `b`, without leaking timing information that
 would allow an attacker to guess one of the values. This is suitable for