Add evo_megamenu_json extension and seed the megamenu

- new project-neutral extension evo_megamenu_json (Evomedien): a Megamenu
  plugin holding entries, columns, items, teaser cards and a featured slot
  as nested records; presentation settings travel with the payload so the
  front end reads behaviour instead of hard-coding it; published as
  page.10.fields.megaMenu, driven by the site settings megamenu.contentUid
  and megamenu.storagePid
- vitec:seed-megamenu fills one element from the live structure: 5 entries,
  26 columns, 95 items, 4 story cards; items without a page yet are flagged
  pending so the front end falls back to the column link
- vitec:debug-sets prints the resolved site set order
- read-only diagnostics: check_megamenu_schema.php, check_typoscript_templates.php
- docs: schema updates run via extension:setup (database:updateschema no
  longer exists in v14), and a JSON Accept header activates headless-mixed,
  which strips every set-added page field

packages/evo_megamenu_json/Configuration/Sets/Megamenu/config.yaml

@@ -0,0 +1,9 @@
+name: evomedien/megamenu
+label: EVO Megamenu (JSON)
+
+# headless has to be loaded before this set: it resets the whole page object
+# (`page < lib.headlessPage`), which would drop the page-level field added
+# here. Declaring the dependency keeps that order without the site
+# configuration having to care about the listing position.
+dependencies:
+  - friendsoftypo3/headless