[cs] support interpolated __cs__

gencs.ml

@@ -1286,6 +1286,8 @@ let configure gen =
 					write w " )";
 				| TCall ({ eexpr = TLocal( { v_name = "__cs__" } ) }, [ { eexpr = TConst(TString(s)) } ] ) ->
 					write w s
+				| TCall ({ eexpr = TLocal( { v_name = "__cs__" } ) }, { eexpr = TConst(TString(s)) } :: tl ) ->
+					Codegen.interpolate_code gen.gcon s tl (write w) (expr_s w) e.epos
 				| TCall ({ eexpr = TLocal( { v_name = "__unsafe__" } ) }, [ e ] ) ->
 					write w "unsafe";
 					expr_s w (mk_block e)