ssh/browser: implement browser client for Tailscale SSH

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>

ssh/browser/dist/placeholder

@@ -0,0 +1,2 @@
+This is here to make sure the dist directory exists for the go:embed command
+in browser.go.