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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tests/unit/style/properties/serialization.rs

@@ -536,11 +536,11 @@ mod shorthand_serialization {
     #[test]
     fn columns_should_serialize_correctly() {
         use style::properties::longhands::column_count::SpecifiedValue as ColumnCount;
-        use style::properties::longhands::column_width::SpecifiedValue as ColumnWidth;
+        use style::values::{Auto, Either};
 
         let mut properties = Vec::new();
 
-        let width = DeclaredValue::Value(ColumnWidth::Auto);
+        let width = DeclaredValue::Value(Either::Second(Auto));
         let count = DeclaredValue::Value(ColumnCount::Auto);
 
         properties.push(PropertyDeclaration::ColumnWidth(width));