Add the artboard dimensions to the Context

Allow nodes to retrieve the dimensions of the artboard, if present, with an Artboard Size node that extracts the width and height from the Context.

@TrueDoctor do you have thoughts or concerns with this idea?

Start by tracing how the Context is defined and how existing nodes extract values from it. Determine how an Artboard Size node should expose the artboard width and height, including the no-artboard case, then verify that nodes can retrieve both dimensions through the Context.

索引モデルが issue の本文から書いたものです。