LoroList indexOf

hi,
i want to insert a new LoroMap into LoroList:

```
// insert a new item after another item
let current=xxx
let index=list.indexOf(current)
let newItem=list.insertContainer(index+1,new LoroMap())
```

how to get index of an item from a LoroList?

or it's safer to call something like this( currently no suck api):
let newItem=list.insertContainerAfter(item,new LoroMap())

