feat: Rewrote inventory handling basing on ESCItems (#417)

Co-authored-by: Dennis Ploeger <develop@dieploegers.de>

game/items/inventory/pen.esc

@@ -0,0 +1,16 @@
+:look
+say player "It's a pen."
+stop
+
+:pickup
+set_global i/r5_pen true
+set_active r5_pen false
+
+:use r5_empty_sheet
+say player "So, let's see..."
+#set_global i/r5_pen false
+inventory_remove r5_pen
+#set_global i/r5_empty_sheet false
+inventory_remove r5_empty_sheet
+#set_global i/r5_filled_sheet true
+inventory_add r5_filled_sheet