LibWeb: Make resolved serialization of CSS display prefer short form

Although we translate e.g `block` to `block flow` for internal use in
the engine, CSS-DISPLAY-3 tells us to use the short form in
serializations for compatibility reasons.

This adds 9 points to our score on https://html5test.com/ :^)

Tests/LibWeb/Text/expected/css/getComputedStyle-display-short-serialization.txt

@@ -0,0 +1,14 @@
+none => none
+block => block
+flow-root => flow-root
+inline => inline
+inline-block => inline-block
+run-in => run-in
+list-item => list-item
+flex => flex
+inline-flex => inline-flex
+grid => grid
+inline-grid => inline-grid
+ruby => ruby
+table => table
+inline-table => inline-table

Tests/LibWeb/Text/input/css/getComputedStyle-display-short-serialization.html

@@ -0,0 +1,32 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const e = document.createElement("div");
+        document.body.appendChild(e);
+        function checkDisplay(display) {
+            e.style.display = display;
+            const computedStyle = getComputedStyle(e);
+            const serialized = computedStyle.display;
+            println(display + " => " + serialized);
+        }
+        for (display of [
+            "none",
+            "block",
+            "flow-root",
+            "inline",
+            "inline-block",
+            "run-in",
+            "list-item",
+            "flex",
+            "inline-flex",
+            "grid",
+            "inline-grid",
+            "ruby",
+            "table",
+            "inline-table",
+        ]) {
+            checkDisplay(display);
+        }
+        e.remove();
+    });
+</script>

Tests/LibWeb/Text/input/include.js

@@ -0,0 +1,15 @@
+var __outputElement = null;
+
+function println(s) {
+    __outputElement.appendChild(document.createTextNode(s + "\n"));
+}
+
+document.addEventListener("DOMContentLoaded", function () {
+    __outputElement = document.createElement("pre");
+    __outputElement.setAttribute("id", "out");
+    document.body.appendChild(__outputElement);
+});
+
+function test(f) {
+    document.addEventListener("DOMContentLoaded", f);
+}