Support `contain: inline-size` property.

[Seirdy]

Relevant specification: CSS Containment Module, Level 3

The validator currently fails to recognize inline-size as a valid value for the contain property. When invoked via the Nu HTML Checker, the CSS validator returns the message:

"CSS: “contain”: “inline-size” is not a “contain” value."

This feature is implemented in WebKit, Firefox, and Chromium.

• WebKit bug 236354
• Mozilla bug 1755565