Utils: Add std::expected implementation

Adds a std::expected implementation that is compatible with >= C++11.

FilePath::fileContents and FilePath::writeFileContents as well as
FilePath::copyFile are changed to return std::expected.

A couple of macros have been added to aid in using the expected types.

An auto test was added showing how to use the library.

Change-Id: Ibe3aecfc1029a0cf13b45bf5184ff03a04a2393b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/coreplugin/loggingviewer.cpp

@@ -652,7 +652,7 @@ void LoggingViewManagerWidget::loadAndUpdateFromPreset()
     if (fp.isEmpty())
         return;
     // read file, update categories
-    const std::optional<QByteArray> contents = fp.fileContents();
+    const Utils::expected_str<QByteArray> contents = fp.fileContents();
     if (!contents) {
         QMessageBox::critical(ICore::dialogParent(),
                               tr("Error"),