manager: allow parent path annotations

- Allows unlimited amount of `..\` or `../` at the start of DestPath string.
- Each of then will go back one folder.
- Similar to how Linux works or Visual Studio project styles.
- This allows us to install/update lwjgl3ify in PrismLauncher.

ModpackUpdater.Manager/Extensions.cs

@@ -67,4 +67,18 @@ public static class Extensions
             return @this.UpdateUrl?.Replace("{ref}", @this.RefTag);
         return @this.UpdateUrl;
     }
+
+    public static string GetDestPath(this InstallAction @this, string installDir)
+    {
+        var path = @this.DestPath;
+        var root = Path.GetPathRoot(path);
+        var start = installDir;
+        while (root.Length == 3 && root.StartsWith(".."))
+        {
+            path = path[root.Length..];
+            root = Path.GetPathRoot(path);
+            start = Path.GetDirectoryName(start);
+        }
+        return Path.Combine(start, path);
+    }
 }