LibWeb: Add naive support for document.querySelectorAll()

This currently returns a JS::Array of elements matching a selector.
The more correct behavior would be to return a static NodeList, but as
we don't have NodeLists right now, that'll be a task for the future.

Libraries/LibWeb/Parser/CSSParser.cpp

@@ -267,6 +267,9 @@ public:
 
     Optional<Selector::SimpleSelector> parse_simple_selector()
     {
+        if (!peek())
+            return {};
+
         if (consume_whitespace_or_comments())
             return {};
 
@@ -658,6 +661,16 @@ private:
     StringView css;
 };
 
+Optional<Selector> parse_selector(const StringView& selector_text)
+{
+    CSSParser parser(selector_text);
+    auto complex_selector = parser.parse_complex_selector();
+    if (!complex_selector.has_value())
+        return {};
+    complex_selector.value().relation = Selector::ComplexSelector::Relation::None;
+    return Selector({ complex_selector.value() });
+}
+
 RefPtr<StyleSheet> parse_css(const StringView& css)
 {
     CSSParser parser(css);