Add `font-style-normal`

Created by: simzikov

When you need to change the font style to italic, you can use font-italic. However, sometimes you need to change it back to normal (say all of your .blockquotes are italic and you need to make one of them normal). We already have font-weight utilities which names start with font-weight. Probably font-style utilities naming should be corrected as well:

font-style-normal font-style-italic