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.css

@@ -0,0 +1,91 @@
+/* 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 "xterm/css/xterm.css";
+
+html {
+    background: #fff;
+    font-family: ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif,  "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
+}
+
+body {
+    margin: 0;
+}
+
+button {
+    font-family: inherit;
+    border: solid 1px #ccc;
+    background: #fff;
+    color: #000;
+    padding: 4px 8px;
+    border-radius: 4px;
+}
+
+#header {
+    background: #f7f5f4;
+    border-bottom: 1px solid #eeebea;
+    padding: 12px;
+    display: flex;
+    align-items: center;
+}
+
+#header h1 {
+    margin: 0;
+    flex-grow: 1;
+}
+
+#header #state {
+    padding: 0 8px;
+    color: #444342;
+}
+
+#peers {
+    box-sizing: border-box;
+    width: 100%;
+    padding: 12px;
+}
+
+.login {
+    text-align: center;
+}
+
+.logout {
+    font-weight: bold;
+}
+
+.peer {
+    display: flex;
+    justify-content: space-between;
+    padding: 2px;
+}
+
+.peer:hover {
+    background: #eee;
+}
+
+.peer .name {
+    font-family: monospace;
+}
+
+.peer .ssh {
+    background-color: #cbf4c9;
+}
+
+.term-container {
+    padding: 12px;
+}
+
+.xterm-viewport.xterm-viewport {
+    scrollbar-width: thin;
+}
+.xterm-viewport::-webkit-scrollbar {
+    width: 10px;
+}
+.xterm-viewport::-webkit-scrollbar-track {
+    opacity: 0;
+}
+.xterm-viewport::-webkit-scrollbar-thumb {
+    min-height: 20px;
+    background-color: #ffffff20;
+}