feature #34880 [Twig][Form] Twig theme for Foundation 6 (Lyssal)

This PR was squashed before being merged into the 5.1-dev branch (closes #34880).

Discussion
----------

[Twig][Form] Twig theme for Foundation 6

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        | -

Hi,

I propose a form theme for Foundation 6. This layout stylizes buttons, percent / money widgets and permits switch inputs adding a 'switch-input' attribute (as for https://symfony.com/blog/new-in-symfony-4-4-bootstrap-custom-switches).

![Foundation_6](https://user-images.githubusercontent.com/10855303/70390292-184cdc80-19ca-11ea-8ad8-090358dd9e35.png)

```php
public function buildForm(FormBuilderInterface $builder, array $options): void
{
    $builder
        // ...
        ->add('checkbox-switch', Type\CheckboxType::class, [
            'attr' => [
                'class' => 'switch-input',
            ],
        ])
    ;
}
```

Commits
-------

e47a134 [Twig][Form] Twig theme for Foundation 6

src/Symfony/Bridge/Twig/CHANGELOG.md

@@ -8,6 +8,8 @@ CHANGELOG
  * removed `transChoice` filter and token
  * `HttpFoundationExtension` requires a `UrlHelper` on instantiation
  * removed support for implicit STDIN usage in the `lint:twig` command, use `lint:twig -` (append a dash) instead to make it explicit.
+ * added form theme for Foundation 6
+ * added support for Foundation 6 switches: add the `switch-input` class to the attributes of a `CheckboxType`
 
 4.4.0
 -----

src/Symfony/Bridge/Twig/Resources/views/Form/foundation_6_layout.html.twig

@@ -0,0 +1,50 @@
+{% extends "form_div_layout.html.twig" %}
+
+{%- block checkbox_row -%}
+    {%- set parent_class = parent_class|default(attr.class|default('')) -%}
+    {%- if 'switch-input' in parent_class -%}
+        {{- form_label(form) -}}
+        {%- set attr = attr|merge({class: (attr.class|default('') ~ ' switch-input')|trim}) -%}
+        {{- form_widget(form) -}}
+        <label class="switch-paddle" for="{{ form.vars.id }}"></label>
+        {{- form_errors(form) -}}
+    {%- else -%}
+        {{- block('form_row') -}}
+    {%- endif -%}
+{%- endblock checkbox_row -%}
+
+{% block money_widget -%}
+    {% set prepend = not (money_pattern starts with '{{') %}
+    {% set append = not (money_pattern ends with '}}') %}
+    {% if prepend or append %}
+        <div class="input-group">
+            {% if prepend %}
+                <span class="input-group-label">{{ money_pattern|form_encode_currency }}</span>
+            {% endif %}
+            {% set attr = attr|merge({class: (attr.class|default('') ~ ' input-group-field')|trim}) %}
+            {{- block('form_widget_simple') -}}
+            {% if append %}
+                <span class="input-group-label">{{ money_pattern|form_encode_currency }}</span>
+            {% endif %}
+        </div>
+    {% else %}
+        {{- block('form_widget_simple') -}}
+    {% endif %}
+{%- endblock money_widget %}
+
+{% block percent_widget -%}
+    {%- if symbol -%}
+        <div class="input-group">
+            {% set attr = attr|merge({class: (attr.class|default('') ~ ' input-group-field')|trim}) %}
+            {{- block('form_widget_simple') -}}
+            <span class="input-group-label">{{ symbol|default('%') }}</span>
+        </div>
+    {%- else -%}
+        {{- block('form_widget_simple') -}}
+    {%- endif -%}
+{%- endblock percent_widget %}
+
+{% block button_widget -%}
+    {% set attr = attr|merge({class: (attr.class|default('') ~ ' button')|trim}) %}
+    {{- parent() -}}
+{%- endblock button_widget %}