LibWeb: Allow font-family names to start with `-`

We achieve this by making properties that accept a custom-ident value
skip the "someone else's vendor prefix" check for values that start with
a `-` character.

This fixes an issue where e.g `font-family: Arial, -apple-system` would
be rejected by the parser completely. We now treat `-apple-system` like
an identifier in such cases.

Also add `valid-types` metadata for the `font-family` property so this
actually works. :^)

Tests/LibWeb/Text/expected/css/font-family-with-dash-prefix.txt

@@ -0,0 +1 @@
+serif, -apple-system

Tests/LibWeb/Text/input/css/font-family-with-dash-prefix.html

@@ -0,0 +1,9 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const e = document.createElement("div");
+        e.style.fontFamily = 'sans-serif';
+        e.style.fontFamily = 'serif, -apple-system';
+        println(e.style.fontFamily);
+    });
+</script>

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -7928,6 +7928,7 @@ Parser::ParseErrorOr<NonnullRefPtr<StyleValue>> Parser::parse_css_value(Property
     m_context.set_current_property_id(property_id);
     Vector<ComponentValue> component_values;
     bool contains_var_or_attr = false;
+    bool const property_accepts_custom_ident = property_accepts_type(property_id, ValueType::CustomIdent);
 
     while (tokens.has_next_token()) {
         auto const& token = tokens.next_token();
@@ -7941,7 +7942,7 @@ Parser::ParseErrorOr<NonnullRefPtr<StyleValue>> Parser::parse_css_value(Property
             if (token.is(Token::Type::Whitespace))
                 continue;
 
-            if (token.is(Token::Type::Ident) && has_ignored_vendor_prefix(token.token().ident()))
+            if (!property_accepts_custom_ident && token.is(Token::Type::Ident) && has_ignored_vendor_prefix(token.token().ident()))
                 return ParseError::IncludesIgnoredVendorPrefix;
         }
 

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -851,7 +851,11 @@
   },
   "font-family": {
     "inherited": true,
-    "initial": "sans-serif"
+    "initial": "sans-serif",
+    "valid-types": [
+      "custom-ident",
+      "string"
+    ]
   },
   "font-size": {
     "inherited": true,