Lint: action_batch.rs compiles without an unused import on wasm32

On main, cargo xtask wasm printed an unused-import warning for json in
crates/engine/src/cmd/fileio/action_batch.rs, and this check reported
it as an error:

    RUSTFLAGS=-Dwarnings cargo check -p vectorcraft-engine --target wasm32-unknown-unknown

Every use of json! in action_batch.rs is in code compiled only for
native targets. The wasm32 version of action_batch uses Value only.

The json import now has the same #[cfg(not(target_arch = "wasm32"))]
attribute as the code that uses it. Native builds import the same
names as before.

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

@@ -5,7 +5,9 @@
 //! Save and Close, or Folder; Override Open / Save / Export. Cited:
 //! <https://helpx.adobe.com/illustrator/desktop/automate-visualize-data/automate-actions/batch-options.html>
 
-use serde_json::{Value, json};
+use serde_json::Value;
+#[cfg(not(target_arch = "wasm32"))]
+use serde_json::json;
 
 use super::super::*;