qtlanguageserver: qtypedjson::reader mismatchs the actual type of variant

Encountered this while testing  tst_qmlls_modules::SemanticHighlightingDelta. Although, the correct variant type (SemanticTokensDelta) is processed in the SemanticHighlightingDeltaHandler:process, the handler matches the first type (SemanticTokens) and doesn't try to match the second type in the variant.

Investigate the method

template<typename... T>
void Reader::handleVariant(std::variant<T...> &el)