feature request: cli completion

[3dyuval]

Most widely used cli allow to generate completions via a top level command.

It would be so cool to have this, so we get this while the cursor is **:

bdg cdp Page**.navigate
navigate | navigateToHistoryEntry

The flow is simple. Wen can use gh completion <shell> to see a complete implementation.

❯ gh completion -h
Generate shell completion scripts for GitHub CLI commands.

When installing GitHub CLI through a package manager, it's possible that
no additional shell configuration is necessary to gain completion support. For
Homebrew, see <https://docs.brew.sh/Shell-Completion>

If you need to set up completions manually, follow the instructions below. The exact
config file locations might vary based on your system. Make sure to restart your
shell before testing whether completions are working.

### bash

First, ensure that you install `bash-completion` using your package manager.

After, add this to your `~/.bash_profile`:

	eval "$(gh completion -s bash)"

### zsh

Generate a `_gh` completion script and put it somewhere in your `$fpath`:

	gh completion -s zsh > /usr/local/share/zsh/site-functions/_gh

Ensure that the following is present in your `~/.zshrc`:

	autoload -U compinit
	compinit -i

Zsh version 5.7 or later is recommended.

### fish

Generate a `gh.fish` completion script:

	gh completion -s fish > ~/.config/fish/completions/gh.fish

### PowerShell

Open your profile script with:

	mkdir -Path (Split-Path -Parent $profile) -ErrorActionSilentlyContinue
	notepad $profile

Add the line and save the file:

	Invoke-Expression -Command $(gh completion -s powershell | Out-String)


USAGE
  gh completion -s <shell>

FLAGS
  -s, --shell string   Shell type: {bash|zsh|fish|powershell}

INHERITED FLAGS
  --help   Show help for command

LEARN MORE
  Use `gh <command> <subcommand> --help` for more information about a command.
  Read the manual at https://cli.github.com/manual
  Learn about exit codes using `gh help exit-codes`
  Learn about accessibility experiences using `gh help accessibility`