fix: plugin will parse fallback until it finds a known value

[calebdwilliams]

This pull request provides a fix for #208 Custom Property as Fallback for Custom Property .

Existing behavior

Currently the library only parses one level of the custom property tree to set a fallback value, so the

:root {
  --primary: tomato;
}
.rule {
  color: var(--override, var(--primary));
}

will yield

:root {
  --primary: tomato;
}
.rule {
  color: var(--primary);
  color: var(--override, var(--primary));
}

New behavior

The above example will now yield

:root {
  --primary: tomato;
}
.rule {
  color: tomato;
  color: var(--override, var(--primary));
}

This should work regardless of how many fallbacks are used.

[calebdwilliams]

See also #240

[calebdwilliams]

@jonathantneal Is there anything else this needs before being merged? My organization's ability to use PostCSS for our purposes kind of hinges on this functionality.

[jonathantneal]

Fantastic work, @calebdwilliams. Thank you!