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/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@tailscale/ssh",
+  "version": "0.0.1",
+  "devDependencies": {
+    "qrcode": "^1.5.0",
+    "xterm": "^4.18.0"
+  },
+  "prettier": {
+    "semi": false,
+    "printWidth": 80
+  }
+}