Auto merge of #15466 - hiei23:master, r=Wafflespeanut

-added parsing and serialization for grid-{row,column}-gap

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I have added an implementation for grid-{row,column}-gap based on the predefined type property suggestion in the description

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15310 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/properties/longhand/position.mako.rs

@@ -229,3 +229,17 @@ ${helpers.predefined_type("object-position",
                               products="gecko",
                               boxed=True)}
 % endfor
+
+${helpers.predefined_type("grid-row-gap",
+                          "LengthOrPercentage",
+                          "computed::LengthOrPercentage::Length(Au(0))",
+                          spec="https://drafts.csswg.org/css-grid/#propdef-grid-row-gap",
+                          animatable=True,
+                          products="gecko")}
+
+${helpers.predefined_type("grid-column-gap",
+                          "LengthOrPercentage",
+                          "computed::LengthOrPercentage::Length(Au(0))",
+                          spec="https://drafts.csswg.org/css-grid/#propdef-grid-column-gap",
+                          animatable=True,
+                          products="gecko")}