[Locale] added some more stubs for the number formatter

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8588, #6045
| License       | MIT
| Doc PR        | n/a

I've used this snippet of code to populate the default values for the en locale:

```php
for ($style = 0; $style <= 8; $style++) {
    $f = new \NumberFormatter('en', $style);
    echo 'array(';
    for ($i = 0; $i <= 17; $i++) {
        echo "'".$f->getSymbol($i)."', ";
    }

    echo "),\n";
}
```