I know some Simmers are opposed to using Moveobjects in their builds, but if you're open to the idea, it's very easy to put lights on fence posts using this build cheat. Just turn on the cheat, find one of the outdoor ground lights that you'd like to use. Place it inside the post, then use the 9 key to raise it up to the top of the post. Here's an example of a build where I've used this method:
https://i.imgur.com/2yA3qbY.png
Hope this is helpful! :)
I do agree it would be nice if we could have snappable fence post lights though, similar to what we had in TS2. Hopefully one day. Also, @xamira99 is correct, we do have a fence that came in a recent free update that has outdoor fence post lights already incorporated into them. It was included with the configurable u-shaped stairs update. The Devs did a livestream that showcased the fence too.
Happy Simming! <3