Auto indent new line after mapping without scalar value

Hi

I'd like to propose some small improvement:
Given

```
mymappings:|
```

with cursor position at |
1.  Invoke enter

Is:

```
mymappings:
|
```

Should:

```
mymappings:
   |
```

Reason: Because the key node is not a scalar it is very likely that a collection will follow.

Thank you.
