Auto merge of #14394 - jcdyer:jcdyer/either-column-width, r=Wafflespeanut

Convert column-width to use Either

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* Converted column-width to use `Either<Length, Auto>`
* Added gecko glue code
* Cleaned up old column-width glue code

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #14350 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because tests already surround the refactored code.
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components/style/properties/properties.mako.rs

@@ -1145,7 +1145,10 @@ impl ComputedValues {
     #[inline]
     pub fn is_multicol(&self) -> bool {
         let style = self.get_column();
-        style.column_count.0.is_some() || style.column_width.0.is_some()
+        match style.column_width {
+            Either::First(_width) => true,
+            Either::Second(_auto) => style.column_count.0.is_some(),
+        }
     }
 
     /// Resolves the currentColor keyword.