Iterate on type safety for `zadd`

Motivation:

Issue #60 called for improving the type safety of the options available for the `zadd` command, and MR !70 made some great headway, but attempted to cram too much into a single enum.

Modifications:

- Break the `RedisSortedSetAddOption.returnChangedCount` value into an additional boolean param

Result:

Using `zadd` should now be more straight forward, while being type safe.