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/lib.rs

@@ -1854,6 +1854,8 @@ mod tests_layerclip;
 #[cfg(test)]
 mod tests_layers;
 #[cfg(test)]
+mod tests_legacy_ai;
+#[cfg(test)]
 mod tests_library;
 #[cfg(test)]
 mod tests_linked_stops;