Implement a MinLength type

Implement an ExtremumLength type which contains all the enumerated
keyword values for min-width, min-height, max-width, and
max-height. Then, implement a MinLength which can be used for min-width
and min-height. So far this just maps to Gecko values.

Refs #13821.

components/style/values/mod.rs

@@ -185,3 +185,11 @@ impl<A: ToComputedValue, B: ToComputedValue> ToComputedValue for Either<A, B> {
         }
     }
 }
+
+// A type for possible values for min- and max- flavors of width,
+// height, block-size, and inline-size.
+define_css_keyword_enum!(ExtremumLength:
+                         "max-content" => MaxContent,
+                         "min-content" => MinContent,
+                         "fit-content" => FitContent,
+                         "fill-available" => FillAvailable);