chore: add missing robot.back method

and keep robot.backward as an alias

libs/robot.lua

@@ -42,7 +42,7 @@ api.create = function(state)
     return ok, err
   end
 
-  robot.backward = function()
+  robot.back = function()
     local ok, err = turtle.back()
 
     if ok then
@@ -51,6 +51,7 @@ api.create = function(state)
 
     return ok, err
   end
+  robot.backward = robot.back
 
   robot.up = function()
     local ok, err = turtle.up()