add .mjs as extension for javascript files in defaultLanguageMapping

Change-Id: I40d43193ba9b7dac38d7a474aad2d739e6c39dc6
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/libs/qmljs/qmljsmodelmanagerinterface.cpp

@@ -165,6 +165,7 @@ ModelManagerInterface::~ModelManagerInterface()
 static QHash<QString, Dialect> defaultLanguageMapping()
 {
     static QHash<QString, Dialect> res{
+        {QLatin1String("mjs"), Dialect::JavaScript},
         {QLatin1String("js"), Dialect::JavaScript},
         {QLatin1String("qml"), Dialect::Qml},
         {QLatin1String("qmltypes"), Dialect::QmlTypeInfo},