LibWeb: Implement CSSRule and CSSStyleDeclaration serialization

There are a handful of FIXME's here, but this seems generally good.
Note that CSS *values* don't get serialized in a spec-compliant way
since we currently rely on StyleValue::to_string() which is ad-hoc.

Userland/Libraries/LibWeb/CSS/CSSImportRule.cpp

@@ -19,4 +19,27 @@ CSSImportRule::~CSSImportRule()
 {
 }
 
+// https://drafts.csswg.org/cssom/#serialize-a-css-rule
+String CSSImportRule::serialized() const
+{
+    StringBuilder builder;
+    // The result of concatenating the following:
+
+    // 1. The string "@import" followed by a single SPACE (U+0020).
+    builder.append("@import "sv);
+
+    // 2. The result of performing serialize a URL on the rule’s location.
+    // FIXME: Look into the correctness of this serialization
+    builder.append("url("sv);
+    builder.append(m_url.to_string());
+    builder.append(')');
+
+    // FIXME: 3. If the rule’s associated media list is not empty, a single SPACE (U+0020) followed by the result of performing serialize a media query list on the media list.
+
+    // 4. The string ";", i.e., SEMICOLON (U+003B).
+    builder.append(';');
+
+    return builder.to_string();
+}
+
 }