feat(lua): add <f-args> to user commands callback (#17522)

Works similar to ex <f-args>. It only splits the arguments if the
command has more than one posible argument. In cases were the command
can only have 1 argument opts.fargs = { opts.args }

runtime/doc/api.txt

@@ -651,6 +651,9 @@ nvim_add_user_command({name}, {command}, {*opts})
                                that contains the following keys:
                                • args: (string) The args passed to the
                                  command, if any |<args>|
+                               • fargs: (table) The args split by unescaped
+                                 whitespace (when more than one argument is
+                                 allowed), if any |<f-args>|
                                • bang: (boolean) "true" if the command was
                                  executed with a ! modifier |<bang>|
                                • line1: (number) The starting line of the