fix plural forms validation

[soukicz]

My previous PR #156 missed some characters in plural forms validation

[oscarotero]

Maybe an easier solution is just check whether the plural form has any letter different than n?
Somethin like this (not tested):

if (preg_match('/[A-Ma-mO-Zo-z]/', str_replace('return ', '', $code)) {
    throw new \InvalidArgumentException('Invalid Plural form: ' . $code);
}

Just an idea, what do you think?

[soukicz]

It would be also better to check it in setter before variable substitution. I will updated it.

[oscarotero]

We can also test the regexp with all possible formulas to be sure that doesn't fail with any valid plural formula.

$languages = Gettext\Languages\Language::getAll();

foreach ($languages as $language) {
    //test $language->formula
}

[soukicz]

I have simplified validation and added testing for predefined plural forms.

[oscarotero]

Great. Thank you 👍