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/dev.go

@@ -0,0 +1,38 @@
+// 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.
+
+package main
+
+import (
+	"log"
+	"net"
+	"strconv"
+
+	esbuild "github.com/evanw/esbuild/pkg/api"
+)
+
+func runDev() {
+	buildOptions, err := commonSetup(devMode)
+	if err != nil {
+		log.Fatalf("Cannot setup: %v", err)
+	}
+	host, portStr, err := net.SplitHostPort(*addr)
+	if err != nil {
+		log.Fatalf("Cannot parse addr: %v", err)
+	}
+	port, err := strconv.ParseUint(portStr, 10, 16)
+	if err != nil {
+		log.Fatalf("Cannot parse port: %v", err)
+	}
+	result, err := esbuild.Serve(esbuild.ServeOptions{
+		Port:     uint16(port),
+		Host:     host,
+		Servedir: "./",
+	}, *buildOptions)
+	if err != nil {
+		log.Fatalf("Cannot start esbuild server: %v", err)
+	}
+	log.Printf("Listening on http://%s:%d\n", result.Host, result.Port)
+	result.Wait()
+}