Breakpoints only allow px

[srsgores]

Using the UI, only px units can be set on breakpoints, which should really all use rem at the very least, if not leverage CSS custom properties via :root. This needs to be addressed so that authors can maintain consistency throughout designs and use custom properties.

In fact, using px across the board causes all kinds of issues in responsive designs and should be avoided -- this also applies to transitions, which should use rem, %, or em. I will create a separate issue for the latter.

[kof]

Here is an issue to follow for that webstudio-is/webstudio#4440