Indent deprecation warning's userland message

Before it was like:

```
  deprecated: Belt.List.filterWithIndex
This function will soon be deprecated. Please, use `List.keepWithIndex` instead.
```

Now it's:

```
  deprecated: Belt.List.filterWithIndex
  This function will soon be deprecated. Please, use `List.keepWithIndex` instead.
```

This helps with the visual but also the parsing of the messages.
Without our super errors system, this'll now look like:

```
deprecated: Belt.List.filterWithIndex
  This function will soon be deprecated. Please, use `List.keepWithIndex` instead.
```

Which is still tasteful

Author: chenglou

parsing/builtin_attributes.ml

@@ -63,7 +63,8 @@ let cat s1 s2 =
   if s2 = "" then s1 else
 #if undefined BS_NO_COMPILER_PATCH then 
     if Clflags.bs_vscode then s1 ^ " " ^ s2
-    else s1 ^ "\n" ^ s2 
+    (* 2 spaces indentation for the next line *)
+    else s1 ^ "\n  " ^ s2 
 #else    
     s1 ^ "\n" ^ s2
 #end