getCurrentBlockIndex returns different numbers depending on how/if the block has been moved

according to the documentation, `getCurrentBlockIndex` should return 0 for the first block:

```
isFirstBlock() {
return this.api.blocks.getCurrentBlockIndex() === 0;
}
// taken from https://editorjs.io/blocks#getcurrentblockindex
```

however, `getCurrentBlockIndex` returns `realindex -1` on initialization and when the block has been moved **down**
and `realindex + 1` when the block has been moved **up**:

this makes it impossible to programmatically insert items after the current bock

# Demo

to demonstrate, @natterstefan (thanks a lot :rocket: ) set up a demo that logs the currentBlockIndex in a custom plugin (on render and on moved)

https://codepen.io/natterstefan/pen/eYNQObo?editors=1111

## initialization

the block using the custom plugin is the third block

on render, `api.blocks.getCurrentBlockIndex()` returns 1 (instead of 2)

## moving up

moving the block **up** to the second position, makes `moved()` log 2 (instead of 1)

moving the block to the first position: logs 1 (instead of 0)

## moving down

moving the block **down** to the second position: logs 0 (instead of 1)

moving the block **down** to the third position: logs 1 (instead of 2)