Add macos-dock-drop-folder-behavior configuration option

This adds a new configuration option that controls whether folders
dropped onto the Ghostty dock icon open in a new tab (default) or
a new window.

The option accepts two values:
- tab: Opens folders in a new tab in the main window (default)
- window: Opens folders in a new window

This is useful for users who prefer window-based workflows over
tab-based workflows when opening folders via drag and drop.

macos/Sources/App/macOS/AppDelegate.swift

@@ -402,11 +402,17 @@ class AppDelegate: NSObject,
         var config = Ghostty.SurfaceConfiguration()
 
         if (isDirectory.boolValue) {
-            // When opening a directory, create a new tab in the main
-            // window with that as the working directory.
-            // If no windows exist, a new one will be created.
+            // When opening a directory, check the configuration to decide
+            // whether to open in a new tab or new window.
             config.workingDirectory = filename
-            _ = TerminalController.newTab(ghostty, withBaseConfig: config)
+            
+            let behavior = ghostty.config.macosDockDropFolderBehavior
+            if behavior == .window {
+                _ = TerminalController.newWindow(ghostty, withBaseConfig: config)
+            } else {
+                // Default to tab behavior
+                _ = TerminalController.newTab(ghostty, withBaseConfig: config)
+            }
         } else {
             // When opening a file, we want to execute the file. To do this, we
             // don't override the command directly, because it won't load the