Given a string, return if a given letter always appears immediately before another given letter.
best_friend("he headed to the store", "h", "e") ➞ True
# All occurences of "h": ["he", "headed", "the"]
# All occurences of "h" have an "e" after it.
# Return True
best_friend("he headed to the store", "h", "e") ➞ True
best_friend("i found an ounce with my hound", "o", "u") ➞ True
best_friend("we found your dynamite", "d", "y") ➞ False