The core unit of data flowing through a mopress build pipeline.
An Item[T] carries a piece of data of type T (e.g. raw text,
a parsed Markdown AST, or rendered HTML), a set of template variables
(vars) that will be available when the item is eventually rendered
through a template, and a target path describing where the item's
output should be written.
Steps (functions of the shape (Item[T]) -> Item[R], optionally
raising or async) transform an Item from one representation to
another while carrying its variables and target path forward, so that
a page's content can flow through a chain of transformations —
parsing, preprocessing, transforming, rendering, and templating — as a
pipeline of Item values.