LibGUI: Add underlines to highlighting

Author: https://github.com/oriko1010
Commit: 6d89f48dd8
Pull-request: https://github.com/SerenityOS/serenity/pull/1429
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/CppSyntaxHighlighter.h

@@ -7,8 +7,9 @@ namespace GUI {
 class CppSyntaxHighlighter final : public SyntaxHighlighter {
 public:
     CppSyntaxHighlighter() {}
-
     virtual ~CppSyntaxHighlighter() override;
+
+    virtual SyntaxLanguage language() const override { return SyntaxLanguage::Cpp; }
     virtual void rehighlight() override;
     virtual void highlight_matching_token_pair() override;
 };