Add has_many input config option for setting the start index

This PR addresses #3680 by adding a `sortable_start` option to set the start index in a `has_many` input. If the option isn't provided, it defaults to 0 to preserve the existing behavior.

The syntax looks like this:

```ruby
f.has_many :things, sortable: :position, sortable_start: 1 do |t|
  ...
end
```


why the `, 10)`?