Add default positioning for window creation

Set default values for x and y if not provided.

libs/window.lua

@@ -53,8 +53,9 @@ end
 local function clamp(v, lo, hi) return (v < lo) and lo or ((v > hi) and hi or v) end
 
 function lib.create(name, w, h, x, y)
-    if not x then error("no x",2) end
-    if not y then error("no x",2) end
+    local sx,sy = term.getSize()
+    if not x then x = sx/2-w/2 end
+    if not y then y = sy/2-h/2 end
     w = math.floor(w + 0.5)
     h = math.floor(h + 0.5)
     x = math.floor(x + 0.5)