Add validation constraint to `type: label`: disallow multiple lines

Now that #3361534: KernelTestBase::$strictConfigSchema = TRUE and BrowserTestBase::$strictConfigSchema = TRUE do not actually strictly validate is in, we'd be able to instantly see how much effect this would have. Let's find out!

Wow, zero violations, really?! Maybe the violations only occur in functional tests? 🤔

Verified again locally that this indeed works correctly by changing core/profiles/standard/config/install/taxonomy.vocabulary.tags.yml to have a multi-line description and running StandardTest. Fails as expected 👍

So let's:

1. Add explicit test coverage.
2. Run the full test suite again.

Oops, yes, I forgot about that 🙈 Done! Probably hundreds of test failures now 😄

Well well … I guess almost nothing in Drupal core uses a multi-line label!

I really thought at least some contact form would trigger this, or a vocabulary description, or a menu description … but no! 😮

And sure enough, e.g. core/profiles/demo_umami/config/install/contact.form.feedback.yml uses a single line!

#10: works for me!

Turns out that taxonomy.vocabulary.*:description's UI actually does not allow multiple lines. So I'm wrong about that one. 🙈
Same for system.menu.*:description! 🙈
Both of these are apparently used in listing UIs and that's why they only allow a single line.

I am right about contact.form.*:message though. And I managed to find one more: field.widget.settings.text_textarea_with_summary:placeholder.

Since there are so few examples: let's fix them here? First pushed failing test coverage, then the fix (i.e. tweaking config schema).

Question though is if changing to text does that mean it can be translated?

Both type: label and type: text are translatable already:

# Human readable string that must be plain text and editable with a text field.
label:
  type: string
  label: 'Label'
  translatable: true

…
# Human readable string that can contain multiple lines of text or HTML.
text:
  type: string
  label: 'Text'
  translatable: true

— core.data_types.schema.yml

and:

function config_translation_config_schema_info_alter(&$definitions) {
  $map = [
    'label' => '\Drupal\config_translation\FormElement\Textfield',
    'text' => '\Drupal\config_translation\FormElement\Textarea',
    'date_format' => '\Drupal\config_translation\FormElement\DateFormat',
    'text_format' => '\Drupal\config_translation\FormElement\TextFormat',
    'mapping' => '\Drupal\config_translation\FormElement\ListElement',
    'sequence' => '\Drupal\config_translation\FormElement\ListElement',
    'plural_label' => '\Drupal\config_translation\FormElement\PluralVariants',
  ];

👍😊