An interface for all dictionary implementations. It defines the most basic
operations on a dictionary: adding words, checking if a word is correct, and
getting a list of suggestions for misspelled words.
Returns a list of String that are the suggestions to any word. If the word
is correctly spelled, then this method could return just that one word, or
it could still return a list of words with similar spellings.
word - the word that we want to get a list of spelling suggestions for.