[Form] Added pluralization support for labels

Adds an options for all field types: `translation_count` an integer that is passed to `transchoice` to allow the pluralization of labels.

By default, it is `1` for all field types, except `FileType` and `ChoiceType` that default to `$multiple ? 2 : 1`.

The templates need to be changed to use `transchoice(translation_count, {"count": translation_count}, translation_domain)`, but the ones still using `trans({}, translation_domain)` will continue to work just fine. You will notice the `count` variable that is added, this is to allow translation strings like this:

    {0} No items{1} One item|]1,Inf] %count% items

As far as I know, everything is tested, but I am not that familiar with forms, perhaps someone should take a look. 

**Heads up**: Even though this PR does not introduce a BC break for the usage itself, it modifies the output of `StubTranslator::transchoice` so quite a few unrelated tests were broken and had to be adjusted.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10108
| License       | MIT
| Doc PR        | symfony/symfony-docs#3810

- [x] write tests
- [x] submit changes to the documentation
you should apply ``transchoice`` only when ``translation_count`` is not ``null``, and keep using ``trans`` otherwise.

And the placeholder is broken. It should be ``%count%`` which is replaced, not ``count`` (which is likely to replace things in real words in English).
Btw, you can omit the placeholder entirely given that the ``|transchoice`` filter is adding it automatically: https://github.com/symfony/symfony/blob/d318e09eef441aa2c5b928a63d4e8c0c74355d20/src/Symfony/Bridge/Twig/Extension/TranslationExtension.php#L106