add player hiding and eternalfile to default code

MANUAL.md

@@ -1096,6 +1096,10 @@ end
 ### Default Uranium Template code
 ```lua
 require('stdlib.color')
+require('stdlib.players')
+P1:hidden(1)
+P2:hidden(2)
+require('stdlib.eternalfile')()
 
 -- define a basic quad
 local quad = Quad()

src/main.lua

@@ -1,4 +1,8 @@
 require('stdlib.color')
+require('stdlib.players')
+P1:hidden(1)
+P2:hidden(2)
+require('stdlib.eternalfile')()
 
 -- define a basic quad
 local quad = Quad()