default_box_create_on_get=False should still create keys on assigment

As reported before in https://github.com/cdgriffith/Box/issues/164#issuecomment-1359179014

The current behavior of `default_box_create_on_get` is not what you would expect and eliminates a large part what makes `default_box` useful in the first place.

In the following case I would still expect `b.foo.bar` to be set to `1`.

```
>>> b = Box(default_box=True, default_box_create_on_get=False)
>>> b.foo.bar
Box({})
>>> b.foo.bar = 1
>>> b.foo.bar
Box({})
>>> b
Box({})
```

Not sure if this is feasible, but a new config option `default_box_create_on_set` could be added to control this behavior.

Start by reproducing the example in the issue with default_box=True and default_box_create_on_get=False, then trace the default_box assignment path. Determine how nested assignment is handled and whether a configuration option is needed; done means b.foo.bar = 1 persists the nested key while reads still avoid creating keys.

Written by the indexing model from the issue text.