ipn/desktop: add a new package for managing desktop sessions on Windows

This PR adds a new package, ipn/desktop, which provides a platform-agnostic
interface for enumerating desktop sessions and registering session callbacks.
Currently, it is implemented only for Windows.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/desktop/sessions_notwindows.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !windows
+
+package desktop
+
+import "tailscale.com/types/logger"
+
+// NewSessionManager returns a new [SessionManager] for the current platform,
+// [ErrNotImplemented] if the platform is not supported, or an error if the
+// session manager could not be created.
+func NewSessionManager(logger.Logf) (SessionManager, error) {
+	return nil, ErrNotImplemented
+}