cmd/tsconnect: add dev-pkg command for two-sided development

Allows imports of the NPM package added by 1a093ef482
to be replaced with import("http://localhost:9090/pkg/pkg.js"), so that
changes can be made in parallel to both the module and code that uses
it (without any need for NPM publishing or even building of the package).

Updates #5415

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

cmd/tsconnect/tsconnect.go

@@ -36,6 +36,8 @@ func main() {
 	switch flag.Arg(0) {
 	case "dev":
 		runDev()
+	case "dev-pkg":
+		runDevPkg()
 	case "build":
 		runBuild()
 	case "build-pkg":