Add IsPortable flag to update manager

src/Velopack.Packaging.Windows/Commands/WindowsPackCommandRunner.cs

@@ -171,6 +171,9 @@ public class WindowsPackCommandRunner : PackageBuilder<WindowsPackOptions>
         var stubName = (Options.PackTitle ?? Options.PackId) + ".exe";
         File.Move(stubPath, Path.Combine(dir.FullName, stubName));
 
+        // create a .portable file to indicate this is a portable package
+        File.Create(Path.Combine(dir.FullName, ".portable")).Close();
+
         await EasyZip.CreateZipFromDirectoryAsync(Log, outputPath, dir.FullName, Utility.CreateProgressDelegate(progress, 40, 100));
         progress(100);
     }

src/Velopack/Locators/IVelopackLocator.cs

@@ -39,6 +39,14 @@ namespace Velopack.Locators
         /// <summary> The release channel this package was built for. </summary>
         public string? Channel { get; }
 
+        /// <summary>
+        /// A flag indicating if this is a portable build, and that the settings should be self-contained in the package.
+        /// On Windows, this is true for portable builds, and false for non-portable builds which were installed by Setup.exe
+        /// On OSX and Linux, this is always false, because settings and application files should be stored in the user's 
+        /// home directory.
+        /// </summary>
+        public bool IsPortable { get; }
+
         /// <summary>
         /// Finds .nupkg files in the PackagesDir and returns a list of ReleaseEntryName objects.
         /// </summary>

src/Velopack/Locators/VelopackLocator.cs

@@ -59,6 +59,9 @@ namespace Velopack.Locators
         /// <inheritdoc/>
         public abstract string? Channel { get; }
 
+        /// <inheritdoc/>
+        public virtual bool IsPortable => false;
+
         /// <inheritdoc/>
         public virtual string? ThisExeRelativePath {
             get {

src/Velopack/Locators/WindowsVelopackLocator.cs

@@ -31,6 +31,10 @@ namespace Velopack.Locators
         /// <inheritdoc />
         public override string? PackagesDir => CreateSubDirIfDoesNotExist(RootAppDir, "packages");
 
+        /// <inheritdoc />
+        public override bool IsPortable =>
+            RootAppDir != null ? File.Exists(Path.Combine(RootAppDir, ".portable")) : false;
+
         /// <inheritdoc />
         public override string? Channel { get; }
 

src/Velopack/UpdateManager.cs

@@ -24,6 +24,9 @@ namespace Velopack
         /// <summary> True if this application is currently installed, and is able to download/check for updates. </summary>
         public virtual bool IsInstalled => Locator.CurrentlyInstalledVersion != null;
 
+        /// <inheritdoc cref="IVelopackLocator.IsPortable" />
+        public virtual bool IsPortable => Locator.IsPortable;
+
         /// <summary> True if there is a local update prepared that requires a call to <see cref="ApplyUpdatesAndRestart(VelopackAsset, string[])"/> to be applied. </summary>
         public virtual bool IsUpdatePendingRestart {
             get {