Add support for CSS property object-fit

[jessewang-arvatosystems]

object-fit is a valid CSS property https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit but the PrettyCSS linter does thinks it is an invalid property.

To test:
https://popcode.org/?snapshot=eea79bbc-98bc-44ef-81ac-a515b6f10630