QmlDesigner: Add setting for Qt 6 projects

Adding
'qt6Project: true'
to the qmlproject file marks the project as Qt 6 'only'.

Task-number: QDS-4734
Change-Id: Ib9d969fd11d409e280734f9bedf759b40470fc61
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

src/plugins/qmlprojectmanager/qmlproject.h

@@ -79,6 +79,7 @@ public:
     Utils::FilePath mainFilePath() const;
 
     bool qtForMCUs() const;
+    bool qt6Project() const;
     void setMainFile(const QString &mainFilePath);
     Utils::FilePath targetDirectory() const;
     Utils::FilePath targetFile(const Utils::FilePath &sourceFile) const;