tailscale: Generate completions from the CLI.

[lgarron]

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---

tailscale command docs for the completion command:

> tailscale completion --help
Shell tab-completion scripts

USAGE
  tailscale completion <shell> [--flags] [--descs]

To load completions:

Bash:

	$ source <(tailscale completion bash)

	# To load completions for each session, execute once:
	# Linux:
	$ tailscale completion bash > /etc/bash_completion.d/tailscale
	# macOS:
	$ tailscale completion bash > $(brew --prefix)/etc/bash_completion.d/tailscale

Zsh:

	# If shell completion is not already enabled in your environment,
	# you will need to enable it.  You can execute the following once:

	$ echo "autoload -U compinit; compinit" >> ~/.zshrc

	# To load completions for each session, execute once:
	$ tailscale completion zsh > "${fpath[1]}/_tailscale"

	# You will need to start a new shell for this setup to take effect.

fish:

	$ tailscale completion fish | source

	# To load completions for each session, execute once:
	$ tailscale completion fish > ~/.config/fish/completions/tailscale.fish

PowerShell:

	PS> tailscale completion powershell | Out-String | Invoke-Expression

	# To load completions for every new session, run:
	PS> tailscale completion powershell > tailscale.ps1
	# and source this file from your PowerShell profile.

The shell scripts and this help message have been adapted from the
Cobra project (https://cobra.dev, https://github.com/spf13/cobra)
under the Apache-2.0 license. Thank you for making these available.


SUBCOMMANDS
  bash        Generate bash shell completion script
  zsh         Generate zsh shell completion script
  fish        Generate fish shell completion script
  powershell  Generate powershell completion script

I've verified that the updated formula install completions for my shell (fish).

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