Don't sort directories first

### Discussed in https://github.com/nvim-tree/nvim-tree.lua/discussions/2311

<div type='discussions-op-text'>

<sup>Originally posted by **kirasok** July  9, 2023</sup>
Is there way to make nvim-tree *not* to sort directories first?

Like this:
```
a/
a.txt
bcd/
bcd.txt
e.txt
rtx/
```

instead of
```
a/
bcd/
rtx/
a.txt
bcd.txt
e.txt
```</div>