cmd/tsconnect: add npm publish workflow

Adds an on-demand GitHub Action that publishes the package to the npm
registry (currently under tailscale-connect, will be moved to
@tailscale/connect once we get control of the npm org).

Makes the package.json for the NPM package be dynamically generated to
have the current Tailscale client version.

Updates #5415

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/package.json.tmpl

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Template for the package.json that is generated by the build-pkg command.
+// The version number will be replaced by the current Tailscale client version
+// number.
+{
+  "author": "Tailscale Inc.",
+  "description": "Tailscale Connect SDK",
+  "license": "BSD-3-Clause",
+  "name": "tailscale-connect",
+  "type": "module",
+  "main": "./pkg.js",
+  "types": "./pkg.d.ts",
+  "version": "AUTO_GENERATED"
+}