Add <noscript> tag & hide map from screenreaders.

[Sardis93]

Summary

Adds a fallback message displayed inside a <noscript> tag within the map canvas <div> for users who have JavaScript disabled in their browser. The message is configurable via the existing
Settings form using a text format widget (supporting text formats such as plain text or filtered HTML).

Changes

• templates/openy-map.html.twig — adds aria-hidden="true" to .openy-map-canvas to hide the interactive map from screen readers, and renders a <noscript> tag inside it when
  #noscript_text is set on the element
• src/Element/OpenYMap.php — reads noscript_text from config, processes it through check_markup(), and passes the result to the render element as #noscript_text
• src/Form/SettingsForm.php — adds a text_format field to the admin settings form so editors can customise the message and choose a text format
• config/install/openy_map.settings.yml — ships a sensible default value (plain_text format)
• config/schema/openy_map.schema.yml — declares noscript_text as a mapping of value (translatable text) and format (string)
• openy_map.install — openy_map_update_8010() backfills the new config key for existing installations

Test plan

• verify the default message appears in source when JS is disabled
• Settings form: change the message and text format, save, verify the updated text appears in source
• With JS enabled: confirm the <noscript> content is not visible to the user