stream-operator prints superfluous newline at end

This is may be nitpicky, but adding a newline at the end - if wanted - is probably less a hassle than to remove it if unwanted.
```
{
  const auto data = R"(array=["v1", "v2", "v3"])"sv;
  auto table = toml::parse(data);	
  std::stringstream ss;
  ss << table;
  REQUIRE(ss.str() == "array = [ 'v1', 'v3' ]\n"sv); // NOTE extra newline at end
}
```