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>

go.mod

@@ -6,6 +6,7 @@ require (
 	filippo.io/mkcert v1.4.3
 	github.com/akutz/memconn v0.1.0
 	github.com/alexbrainman/sspi v0.0.0-20210105120005-909beea2cc74
+	github.com/andybalholm/brotli v1.0.3
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
 	github.com/aws/aws-sdk-go-v2 v1.11.2
 	github.com/aws/aws-sdk-go-v2/config v1.11.0
@@ -16,6 +17,7 @@ require (
 	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf
 	github.com/creack/pty v1.1.17
 	github.com/dave/jennifer v1.4.1
+	github.com/evanw/esbuild v0.14.39
 	github.com/frankban/quicktest v1.14.0
 	github.com/go-ole/go-ole v1.2.6
 	github.com/godbus/dbus/v5 v5.0.6