Supporting strict property value expression for '/' token in css

[c9s]

CSS allows / to appear in property values as a way of separating numbers. Since SassScript is an extension of the CSS property syntax, it must support this, while also allowing / to be used for division.

When user enables 'strict' mode, @use strict; then the expression will only be evaluated when they're in parenthesis. when strict mode is not enabled, we just behave like SASS.

Related to #17