Tab indentation behavior for nested list item

# Current behavior

Insert tab after `- `

Before inserting tab(`|` presents cursor position)
```md
- item1
- |item2
```

After inserting tab
```md
- item1
-   |item2
```


# Expected behavior

Insert tab before `- `

Before inserting tab
```md
- item1
- |item2
```

After inserting tab
```md
- item1
  - |item2
```
