Part of #868: Save As .ai offers Legacy (version 7), with Transparent Images and an artboard range

aiVersion 7 saves through the legacy writer, one file per artboard; range picks the artboards
and document.save writes them again. Appearances are expanded first. Transparent images
(see-through pixels, opacity or a blending mode) are flattened with the art under them into
opaque images clipped to where they change it (transparentImages "flatten", the default), or
left out ("omit"); both warn.

crates/engine/src/cmd/fileio/mod.rs

@@ -27,6 +27,7 @@ mod encode;
 pub mod eps;
 mod export;
 pub(crate) mod imagemap;
+mod legacy_ai;
 mod load;
 pub(crate) use load::{MAX_RASTER_ALLOC, too_large_note};
 mod metafile;