cmd/tsconnect: initial scaffolding for Tailscale Connect browser client

Runs a Tailscale client in the browser (via a WebAssembly build of the
wasm package) and allows SSH access to machines. The wasm package exports
a newIPN function, which returns a simple JS object with methods like
start(), login(), logout() and ssh(). The golang.org/x/crypto/ssh
package is used for the SSH client.

Terminal emulation and QR code renedring is done via NPM packages (xterm
and qrcode respectively), thus we also need a JS toolchain that can
install and bundle them. Yarn is used for installation, and esbuild
handles loading them and bundling for production serving.

Updates #3157

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

cmd/tsconnect/tsconnect.go

@@ -0,0 +1,60 @@
+// 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.
+
+// The tsconnect command builds and serves the static site that is generated for
+// the Tailscale Connect JS/WASM client. Can be run in 3 modes:
+// - dev: builds the site and serves it. JS and CSS changes can be picked up
+//   with a reload.
+// - build: builds the site and writes it to dist/
+// - serve: serves the site from dist/ (embedded in the binary)
+package main // import "tailscale.com/cmd/tsconnect"
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+)
+
+var (
+	addr = flag.String("addr", ":9090", "address to listen on")
+)
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+	if len(flag.Args()) != 1 {
+		flag.Usage()
+	}
+
+	switch flag.Arg(0) {
+	case "dev":
+		runDev()
+	case "build":
+		runBuild()
+	case "serve":
+		runServe()
+	default:
+		log.Printf("Unknown command: %s", flag.Arg(0))
+		flag.Usage()
+	}
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr, `
+usage: tsconnect {dev|build|serve}
+`[1:])
+
+	flag.PrintDefaults()
+	fmt.Fprintf(os.Stderr, `
+
+tsconnect implements development/build/serving workflows for Tailscale Connect.
+It can be invoked with one of three subcommands:
+
+- dev: Run in development mode, allowing JS and CSS changes to be picked up without a rebuilt or restart.
+- build: Run in production build mode (generating static assets)
+- serve: Run in production serve mode (serving static assets)
+`[1:])
+	os.Exit(2)
+}