Add workspace_auto_back_and_forth command

This is the only missing piece - other code regarding this functionality has
already been ported from pre-wlroots source.

sway/commands.c

@@ -106,6 +106,7 @@ static struct cmd_handler handlers[] = {
 	{ "output", cmd_output },
 	{ "seat", cmd_seat },
 	{ "workspace", cmd_workspace },
+	{ "workspace_auto_back_and_forth", cmd_ws_auto_back_and_forth },
 };
 
 static struct cmd_handler bar_handlers[] = {

sway/commands/ws_auto_back_and_forth.c

@@ -0,0 +1,12 @@
+#include <string.h>
+#include <strings.h>
+#include "sway/commands.h"
+
+struct cmd_results *cmd_ws_auto_back_and_forth(int argc, char **argv) {
+	struct cmd_results *error = NULL;
+	if ((error = checkarg(argc, "workspace_auto_back_and_forth", EXPECTED_EQUAL_TO, 1))) {
+		return error;
+	}
+	config->auto_back_and_forth = !strcasecmp(argv[0], "yes");
+	return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+}

sway/meson.build

@@ -52,6 +52,7 @@ sway_sources = files(
 	'commands/split.c',
 	'commands/swaybg_command.c',
 	'commands/workspace.c',
+	'commands/ws_auto_back_and_forth.c',
 
 	'commands/bar/activate_button.c',
 	'commands/bar/binding_mode_indicator.c',