Fix token build breakpoints

[spiess-demos]

This fix is also applied to the 0.3.x branch in #1011

We cannot use Css variables within media queries. As we do not need to override breakpoints anyway, at least for now, the primitive px value is taken instead.

The change leads to the file tokens/dist/tailwind/breakpoints.js looking like this:

module.exports = {
  "sm": "620px",
  "md": "904px",
  "lg": "1200px",
  "xl": "1400px"
};

instead of this:

module.exports = {
  "sm": "var(--dp-breakpoints-sm, 620px)",
  "md": "var(--dp-breakpoints-md, 904px)",
  "lg": "var(--dp-breakpoints-lg, 1200px)",
  "xl": "var(--dp-breakpoints-xl, 1400px)"
};

[sakutademos]

it works, lgtm!