bug#59920: [PATCH] Make checkdoc warn if not using lexical-binding

[Eli Zaretskii]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Sat, 10 Dec 2022 15:26:03 -0800
> Cc: 59920@debbugs.gnu.org
> 
> +(defcustom checkdoc-lexical-binding-flag t
> +  "Non-nil means generate warnings if file is not using lexical binding.
> +See Info node `(elisp) Converting to Lexical Binding' for more."
> +  :type 'boolean
> +  :version "29.1")

Should be "30.1", I guess?

> +                 (checkdoc-create-error
> +                  "The first line should end with \"-*- lexical-binding: t 
> -*-\""

Is this accurate?  IOW, does the lexical-binding cookie have to be the
last on the first line?

Otherwise, LGTM, thanks.