JavaScript: use this as default _root only in top-level types

See the same change in Lua: 1a0b2c0

This would have fixed the `NestedTypesImport` test for JavaScript if it
weren't for the missing import of `NestedTypes3` it uses (in other
words, this test still fails only because of
kaitai-io/kaitai_struct#703).

shared/src/main/scala/io/kaitai/struct/languages/JavaScriptCompiler.scala

@@ -97,7 +97,11 @@ class JavaScriptCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
     out.inc
     out.puts("this._io = _io;")
     out.puts("this._parent = _parent;")
-    out.puts("this._root = _root || this;")
+    if (name == rootClassName) {
+      out.puts("this._root = _root || this;")
+    } else {
+      out.puts("this._root = _root;")
+    }
 
     if (isHybrid)
       out.puts("this._is_le = _is_le;")