CSS property `overflow-wrap` not included in CssSchema definition list

[kfritz]

The CssSchema class provides a map of CSS property names and values. This list is extensive, but missing some of the more recent additions to CSS.

I would appreciate considering the overflow-wrap property for inclusion in this list. This CSS property directs browsers on how to insert line breaks when text would overflow its box. If the maintainers find this agreeable, I can provide a PR for consideration.