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/src/index.js

@@ -0,0 +1,26 @@
+// 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.
+
+import "./wasm_exec"
+import wasmUrl from "./main.wasm"
+import { notifyState, notifyNetMap, notifyBrowseToURL } from "./notifier"
+import { sessionStateStorage } from "./js-state-store"
+
+const go = new window.Go()
+WebAssembly.instantiateStreaming(
+  fetch(`./dist/${wasmUrl}`),
+  go.importObject
+).then((result) => {
+  go.run(result.instance)
+  const ipn = newIPN({
+    // Persist IPN state in sessionStorage in development, so that we don't need
+    // to re-authorize every time we reload the page.
+    stateStorage: DEBUG ? sessionStateStorage : undefined,
+  })
+  ipn.run({
+    notifyState: notifyState.bind(null, ipn),
+    notifyNetMap: notifyNetMap.bind(null, ipn),
+    notifyBrowseToURL: notifyBrowseToURL.bind(null, ipn),
+  })
+})