qmljs: reduce used threads

Change-Id: I8f27037d0cfefd65f1ac060e1505328ea705a670
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/qmljs/qmljsmodelmanagerinterface.h

@@ -133,6 +133,7 @@ public:
 
     QmlJS::Snapshot snapshot() const;
     QmlJS::Snapshot newestSnapshot() const;
+    QThreadPool *threadPool();
 
     void activateScan();
     void updateSourceFiles(const QStringList &files,
@@ -287,6 +288,7 @@ private:
     Utils::FutureSynchronizer m_futureSynchronizer;
 
     bool m_indexerDisabled = false;
+    QThreadPool m_threadPool;
 };
 
 } // namespace QmlJS