Provide let bindings to meta attributes

[eWert-Online]

Meta Attributes on Pages and Components should be extended in a way, that functions do not get evaluated until the end. Only after the body got rendered, the provided functions get evaluated - recieving the top level let bindings as input.

page BlogPost(
  label: "Blog Post",
  displayName: fun (title) -> "Blog - " ++ title,
) {
  let title = #String(label: "Title");
}