Add Prettyblocks MegaMenu blocks and assembly logic

[CySSoO]

Motivation

• Provide a 100% Prettyblocks-compatible MegaMenu composed of multiple blocks (parent + children) so nested repeaters/items are not used and the hierarchy is managed by parent ID relations.

Description

• Introduces four new Prettyblocks block types and templates: megamenu_item, megamenu_column, megamenu_item_link, and megamenu_item_image and allowlists their templates (views/templates/hook/prettyblocks/prettyblock_megamenu_*.tpl).
• Adds block definitions and configuration fields for the new blocks inside src/Service/EverblockPrettyBlocks.php and registers the templates for rendering in the Prettyblocks UI.
• Implements server-side assembly logic in everblock.php via hookBeforeRenderingEverblockMegamenuItem, plus helpers (getPrettyblocksByCode, getPrettyblocksChoicesByCode, normalizePrettyblocksRelationId, normalizePrettyblocksLink, sortPrettyblocksByOrder) to collect/filter/normalize child blocks by parent ID and order them for the parent template.
• Registers a new before-rendering hook (beforeRenderingEverblockMegamenuItem) so the parent block can receive assembled columns via $block.extra, and updates the module hook registration and the EverblockPrettyBlocks::BEFORE_RENDERING_HOOKS list.

Testing

• No automated tests were executed for this change in the rollout; files were created and committed locally and the code was added to the module (commit message: Add Prettyblocks mega menu block set).

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Codex Task