Support for additional units like em, rem, vh, … in x-collapse.min modifier

[rungta]

The x-collapse .min modifier is great for partially truncating content. However, we are currently limited to specifying a single pixel value. A lot of use-cases involve designs where the amount of visible content is more dynamic — based on the font size, or viewport breakpoint, etc. So it would be useful to be able to specify x-collapse.min.4rem or x-collapse.min.50vh. For even more flexibility, if the min modifier could take in a CSS variable then we can dynamically alter the value based on media queries, in JS, etc.

(Also see this related request: #2984)