Windows: Make Direct3D 12 the default RD driver for new projects

For compatibility reasons, `vulkan` is kept as the technical default for the
setting, but projects created in Godot 4.6 and later will set `d3d12` as the
driver automatically.

Users upgrading to 4.6 are recommended to adopt `d3d12` as a default if it
fits their needs.

Enable `d3d12=yes` by default in SCons so that custom Windows builds include
Direct3D 12 support.

editor/editor_node.cpp

@@ -7884,6 +7884,7 @@ void EditorNode::notify_settings_overrides_changed() {
 HashMap<String, Variant> EditorNode::get_initial_settings() {
 	HashMap<String, Variant> settings;
 	settings["physics/3d/physics_engine"] = "Jolt Physics";
+	settings["rendering/rendering_device/driver.windows"] = "d3d12";
 	return settings;
 }