feat: Add rake task to generate TypeScript types

[tvdeyen]

Frontend apps consuming the API have no types for the elements and page layouts a host app defines, so a typo in an ingredient role or a page layout name only shows up at runtime. bin/rails alchemy:json_api:generate_types generates a .d.ts file from the app's elements.yml and page_layouts.yml that describes the output of deserialize, so frontends can write deserialize<AlchemyPage>(json) and narrow on page.page_layout and element.name, with ingredients narrowed by role, select values as literal unions and nested elements restricted to nestable_elements.

The attribute types are declared next to the attributes in each serializer via typelize, because neither the database schema nor Alchemy's models know them: ingredient fields live in the untyped data JSON column and many attributes are computed in the serializers. A spec fails whenever a serializer attribute has no declared type (or a declaration outlives its attribute), so the types cannot silently drift from the API. Relationship types are derived from the serializers, including lazy_load_data, which the deserializer turns into null unless the relationship was included.

Typelizer was considered, but it has no jsonapi-serializer adapter and generates one type per serializer, so it cannot produce the per-definition types that are the point of this feature. typelize uses the same attribute: "type" shape, which keeps a later switch mechanical.

The dummy app's news element carried rss_title/rss_description, which Alchemy's IngredientDefinition rejects, so they are removed in a separate commit.