style: Add 'arguments' styling rule.
* guix/scripts/style.scm (unquote->ungexp, gexpify-argument-value)
(quote-argument-value, gexpify-argument-tail)
(gexpify-package-arguments): New procedures.
(%gexp-keywords): New variable.
(%options): Add "arguments" case for 'styling-procedure.
(show-stylings): Update.
* tests/style.scm ("gexpify arguments, already gexpified")
("gexpify arguments, non-gexp arguments, margin comment")
("gexpify arguments, phases and flags")
("gexpify arguments, append arguments")
("gexpify arguments, substitute-keyword-arguments")
("gexpify arguments, append substitute-keyword-arguments"): New tests.
* doc/guix.texi (package Reference): For 'arguments', add compatibility
note and link to 'guix style'.
(Invoking guix style): Document the 'arguments' styling rule.
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