style: Add '--styling' option.
* guix/scripts/style.scm (format-package-definition): New procedure.
(%options, show-help): Add "--styling".
(%default-options): Add 'styling-procedure'.
(guix-style): Honor it.
* tests/style.scm (with-test-package)
("input labels, 'safe' policy")
("input labels, 'safe' policy, nothing changed")
("input labels, margin comment")
("input labels, margin comment on long list")
("input labels, line comment")
("input labels, modify-inputs and margin comment"): Pass "-S inputs".
* etc/indent-code.el: Remove.
* doc/contributing.texi (Formatting Code): Mention "guix style" instead
of "etc/indent-code.el".
(Submitting Patches): Add item for "guix style".
* doc/guix.texi (Invoking guix style): Document "-S" and update.
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