cmd/tailscale/jsonoutput: hoist jsonoutput out of cli package (#20591)

Flatten the cmd/tailscale package hierarchy by extracting the
jsonoutput package out of the cmd/tailscale/cli package.

Updates #cleanup

Change-Id: I92f80db75b0328e82f1596b6a42f6f6ef5a94bfa

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/tailscale/jsonoutput/example_responseenvelope_test.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package jsonoutput_test
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"tailscale.com/cmd/tailscale/jsonoutput"
+)
+
+type Hello struct {
+	jsonoutput.ResponseEnvelope
+	Greeting string
+}
+
+func ExampleResponseEnvelope() {
+	hi := Hello{
+		ResponseEnvelope: jsonoutput.ResponseEnvelope{SchemaVersion: "1"},
+		Greeting:         "Hello, world",
+	}
+	out, err := json.MarshalIndent(hi, "", "    ")
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("%s\n", out)
+	// Output:
+	// {
+	//     "SchemaVersion": "1",
+	//     "Greeting": "Hello, world"
+	// }
+}