Superstition is unjustified (or unverifiable) belief that one action will lead to another. An example is the old wives' tale that if a black cat crosses your path, something awful is about to happen. Foreshadowing is a literary device that hints at a future literary event.
Thus, superstition and foreshadowing aren't really similar. However, superstition can create foreshadowing.
First, consider Chekhov's Law (of writing):
Remove everything that has no relevance to the story. If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there.
In Julius Caesar, he's told to "Beware the Ides of March!." We intuitively understand (even if we've never heard of Chekhov's Law) that whatever we're told in a play or narrative is important to the story. Thus, the superstition of the characters and Caesar himself serve as foreshadowing in this play.
Keep in mind, though, that generally speaking, a character can be superstitious and believe whatever a fortuneteller says and none of it come true; this does not create foreshadowing. It just makes the character look a bit loony, not to mention gullible.