Pretty-print values in the REPL

Pretty-print values in the REPL by printing each item in a list or
attrset on a separate line. When possible, single-item lists and
attrsets are printed on one line, as long as they don't contain a nested
list, attrset, or thunk.

Before:
```
{ attrs = { a = { b = { c = { }; }; }; }; list = [ 1 ]; list' = [ 1 2 3 ]; }
```

After:
```
{
  attrs = {
    a = {
      b = {
        c = { };
      };
    };
  };
  list = [ 1 ];
  list' = [
    1
    2
    3
  ];
}
```

src/libcmd/repl.cc

@@ -101,7 +101,8 @@ struct NixRepl
             .ansiColors = true,
             .force = true,
             .derivationPaths = true,
-            .maxDepth = maxDepth
+            .maxDepth = maxDepth,
+            .prettyIndent = 2
         });
     }
 };