LibWeb/CSS: Add dump_string() method to TokenStream

This is to support error reporting. It's not the most elegant way of
getting the source CSS, but it works.

Libraries/LibWeb/CSS/Parser/Token.h

@@ -187,3 +187,11 @@ private:
 };
 
 }
+
+template<>
+struct AK::Formatter<Web::CSS::Parser::Token> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder& builder, Web::CSS::Parser::Token const& token)
+    {
+        return Formatter<StringView>::format(builder, token.to_string());
+    }
+};

Libraries/LibWeb/CSS/Parser/TokenStream.h

@@ -189,6 +189,13 @@ public:
         }
     }
 
+    String dump_string()
+    {
+        // FIXME: The whitespace is only needed because we strip it when parsing property values. Remove it here once
+        //        we stop doing that.
+        return MUST(String::join(" "sv, m_tokens));
+    }
+
 private:
     // https://drafts.csswg.org/css-syntax/#token-stream-tokens
     Span<T const> m_tokens;