Backport: js-indent-align-list-continuation: Make variable safe

* lisp/progmodes/js.el (js-indent-align-list-continuation): Indicate
variable is safe as a file-local variable.  This fixes the
js-indent-align-list-continuation-nil test when run with make.

(cherry picked from commit dd319f27)

lisp/progmodes/js.el

@@ -479,6 +479,7 @@ This applies to function movement, marking, and so on."
   "Align continuation of non-empty ([{ lines in `js-mode'."
   :version "26.1"
   :type 'boolean
+  :safe 'booleanp
   :group 'js)
 
 (defcustom js-comment-lineup-func #'c-lineup-C-comments