cmd/tsconnect: pin yarn and node

Adds a tool/yarn helper script that uses specific versions of yarn and
node, downloading them if necessary.

Modeled after tool/go (and the yarn and node Redo scripts from the
corp repo).

Also allows the path to yarn to be overidden (in case the user does not
want to use this script) and always pipes yarn output (to make debugging
and viewing of process easier).

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/common.go

@@ -97,11 +97,10 @@ func buildWasm(dev bool) error {
 // installJSDeps installs the JavaScript dependencies specified by package.json
 func installJSDeps() error {
 	log.Printf("Installing JS deps...\n")
-	stdoutStderr, err := exec.Command("yarn").CombinedOutput()
-	if err != nil {
-		log.Printf("yarn failed: %s", stdoutStderr)
-	}
-	return err
+	cmd := exec.Command(*yarnPath)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
 }
 
 // EsbuildMetadata is the subset of metadata struct (described by