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/dev-pkg.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"log"
+)
+
+func runDevPkg() {
+	buildOptions, err := commonPkgSetup(devMode)
+	if err != nil {
+		log.Fatalf("Cannot setup: %v", err)
+	}
+	runEsbuildServe(*buildOptions)
+}