feature #32061 Add new Form WeekType (dFayet)

This PR was merged into the 4.4 branch.

Discussion
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Add new Form WeekType

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #32029
| License       | MIT
| Doc PR        | <!--symfony/symfony-docs#...--> coming soon

----
#### Update

After the first try, I've updated the field to have more options, and be more "straight".
The field acts like the `DateTimeType` or `TimeType`,  various fields type (pure text, html5 type, select boxes), data validation, ....

For that I took the choice to update the `DateTimeToStringTransformer` and `DateTimeToArrayTransformer` to make them work with weeks format.

I was not sure if it was better to update them or create new ones, WDYT?

Before addind tests and docs, it would be nice to have your first thoughts/comments 😊

Do you need/want a small test repo?

Commits
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c4a2f026e0 Add new Form WeekType

Author: fabpot

Resources/views/Form/week_widget.html.php

@@ -0,0 +1,14 @@
+<?php if ($widget == 'single_text'): ?>
+    <?php echo $view['form']->block($form, 'form_widget_simple'); ?>
+<?php else: ?>
+    <?php $vars = $widget == 'text' ? ['attr' => ['size' => 1]] : [] ?>
+    <div <?php echo $view['form']->block($form, 'widget_container_attributes') ?>>
+        <?php
+            // There should be no spaces between the colons and the widgets, that's why
+            // this block is written in a single PHP tag
+            echo $view['form']->widget($form['year'], $vars);
+            echo '-';
+            echo $view['form']->widget($form['week'], $vars);
+        ?>
+    </div>
+<?php endif ?>