feature: improve completion default shells, add :cobra format

[daeho-ro]

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This is based on #21287, .

While working on shell completion support, I noticed that when the same packages e.g. cobra, click and clap, are used across mutiple formulae, providing parameter formats with default shells makes the user experience much simpler.

So the goal of this PR is to improve readability by extracting some logic into private methods, and to simplify the DSL by providing more predefined types like :cobra and re-organize the order of shell parameter formats alphabetically.

[MikeMcQuaid]

Looks good thanks again!

[Copilot]

Pull request overview

This PR refactors the generate_completions_from_executable method to improve code maintainability and adds support for the :cobra shell completion format. The refactoring extracts the shell parameter generation logic into a private method and introduces format-specific default shell lists.

Key Changes:

• Added :cobra shell parameter format with default shells including PowerShell
• Refactored shell parameter generation logic into private completion_shell_parameter method
• Reorganized documentation examples alphabetically by format type

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[daeho-ro]

I have noticed that the powershell change can break the current status, so keep it same as original.

[MikeMcQuaid]

Would be nicer to keep this non-nilable and assign in the method definition.