cmd/tailscale: add shell tab-completion

The approach is lifted from cobra: `tailscale completion bash` emits a bash
script for configuring the shell's autocomplete:

    . <( tailscale completion bash )

so that typing:

    tailscale st<TAB>

invokes:

    tailscale completion __complete -- st

RELNOTE=tailscale CLI now supports shell tab-completion

Fixes #3793

Signed-off-by: Paul Scott <paul@tailscale.com>

cmd/tailscale/cli/ffcomplete/complete_omit.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build go1.19 && ts_omit_completion
+
+package ffcomplete
+
+import (
+	"flag"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+func Inject(root *ffcli.Command, hide func(*ffcli.Command), usageFunc func(*ffcli.Command) string) {}
+
+func Flag(fs *flag.FlagSet, name string, comp CompleteFunc)     {}
+func Args(cmd *ffcli.Command, comp CompleteFunc) *ffcli.Command { return cmd }