Block Delete Permission

1. Describe a problem.
I cannot prevent the ToolBar Tunnel button delete blocks, or the backspace key from deleting blocks.
2. Describe the solution you'd like. Mockups are welcome.

why not add if statement in:

https://github.com/codex-team/editor.js/blob/f7368edee3fd5af32a6a9e9903bb9a00a94cff68/src/components/block-tunes/block-tune-delete.ts#L90-L119

It will be:
```typescript
public handleClick(event: MouseEvent): void {
if (!this.api.blocks.canDelete()) {
return;
}

/**
* if block is not waiting the confirmation, subscribe on block-settings-closing event to reset
* otherwise delete block
*/
if (!this.needConfirmation) {
this.setConfirmation(true);

/**
* Subscribe on event.
* When toolbar block settings is closed but block deletion is not confirmed,
* then reset confirmation state
*/
this.api.events.on('block-settings-closed', this.resetConfirmation);
} else {
/**
* Unsubscribe from block-settings closing event
*/
this.api.events.off('block-settings-closed', this.resetConfirmation);

this.api.blocks.delete();
this.api.toolbar.close();
this.api.tooltip.hide();

/**
* Prevent firing ui~documentClicked that can drop currentBlock pointer
*/
event.stopPropagation();
}
}
```

3. Are there any alternatives?
In some scenarios that require permission control, some blocks may not be deleted, or only the specified person is allowed to delete.