PowerShell Completion should not impact global namespace

chezmoi PowerShell Completion missing Native parameter #5133 uses cobra to generate PowerShell completion. Cobra exposes a function and filter to PowerShell global namespace. This should be avoided because it prevents lazy loading of shell completion and could create name collision.

Recommend moving the globals inside the completion [scriptblock].

https://github.com/spf13/cobra/issues/2437 is a related issue with the Native parameter.

Describe the bug

chezmoi completion powershell produces:

# powershell completion for chezmoi                              -*- shell-script -*-

function __chezmoi_debug {
    if ($env:BASH_COMP_DEBUG_FILE) {
        "$args" | Out-File -Append -FilePath "$env:BASH_COMP_DEBUG_FILE"
    }
}

filter __chezmoi_escapeStringWithSpecialChars {
    $_ -replace '\s|#|@|\$|;|,|''|\{|\}|\(|\)|"|`|\||<|>|&','`$&'
}

[scriptblock]${__chezmoiCompleterBlock} = {
    param(
            $WordToComplete,
            $CommandAst,
            $CursorPosition
        )

<snip>

}

Register-ArgumentCompleter -CommandName 'chezmoi' -ScriptBlock ${__chezmoiCompleterBlock}

## To reproduce

```powershell
pwsh --noprofile
chezmoi completion powershell

Expected behavior

# powershell completion for chezmoi                              -*- shell-script -*-

[scriptblock]${__chezmoiCompleterBlock} = {
    param(
            $WordToComplete,
            $CommandAst,
            $CursorPosition
        )

    function __chezmoi_debug {
        if ($env:BASH_COMP_DEBUG_FILE) {
            "$args" | Out-File -Append -FilePath "$env:BASH_COMP_DEBUG_FILE"
        }
    }

    filter __chezmoi_escapeStringWithSpecialChars {
        $_ -replace '\s|#|@|\$|;|,|''|\{|\}|\(|\)|"|`|\||<|>|&','`$&'
    }

<snip>
}

Register-ArgumentCompleter -CommandName 'chezmoi' -ScriptBlock ${__chezmoiCompleterBlock}