Use initial for @property fallbacks instead of

[adamwathan]

This PR updates the way we generate @property fallbacks for Firefox to use initial instead of for properties that are defined with no initial value:

Before:

@property --tw-gradient-via-stops {
  syntax: "*";
  inherits: false;
}

@supports (-moz-orient: inline) {
 * {
   --tw-gradient-via-stops: ;
 }
}

After:

@property --tw-gradient-via-stops {
  syntax: "*";
  inherits: false;
}

@supports (-moz-orient: inline) {
 * {
   --tw-gradient-via-stops: initial;
 }
}

This is necessary because we have code in our CSS that depends on variables like --tw-gradient-via-stops resolving to an invalid value to trigger a fallback:

.from-red-500 {
  --tw-gradient-from: var(--color-red-500, #ef4444);
  --tw-gradient-stops: var(--tw-gradient-via-stops, var(--tw-gradient-from) var(--tw-gradient-from-position), var(--tw-gradient-to) var(--tw-gradient-to-position));
}

If --tw-gradient-via-stops resolves to , then --tw-gradient-stops will also resolve to instead of falling back to the fallback value, which prevents 2-stop gradients from working in Firefox at the moment.

Fixes #13948.