Set placeholder attribute on day, month, year fields

According to the Symphony docs, it's possible to pass an array to the placeholder attribute to set a placeholder on the day/month/year input fields of a date type:

> Alternatively, you can use an array that configures different placeholder values for the year, month and day fields:

https://symfony.com/doc/current/reference/forms/types/date.html#placeholder

I cannot get this to work in Bolt Forms. Is it supported?

My syntax might be incorrect; I have tried:

```
placeholder:
year: 'yyyy'
month: 'mm '
day: 'dd'
```

`placeholder: { year: 'yyyy', month: 'mm', day: 'dd' }`
`placeholder: [ year: 'yyyy', month: 'mm', day: 'dd' ]`