Add DELEX command with conditional predicates (#4607)

Conditional DEL command for strings.

    DELEX key <IFEQ | IFNE> value

It can only delete strings, which is O(1) so it's marked as FAST.

DELEX IFEQ is equivalent to the existing DELIFEQ command. The DELEX name
and syntax were chosen for compatibility with Redis.

Closes #4596.

Signed-off-by: lightsigma96 <8f34yashjadhav@gmail.com>