Allow fallback values for stitches css variables

[joseDaKing]

To use fallback values with raw CSS variables is possible, but the problem with this approach is that theme mapping cannot work.

// This works
css({
  "--size": "32px",
  width: "var(--size, auto)"
});

Theme mapping will only work with stitches CSS variables, but stitches CSS variables don't support fallback values. The syntax for stitches CSS variables with fallback should look like this.

css({
  $$size: "$32", // The size variable will be mapped to a scale
  width: "$$size($16)" // Will use a theme variable that is mapped th width property as fallback value
});