types(joint-core): extract IgnoreEmptyAttributesCallback type alias

Name the predicate signature for `Cell.ExportOptions.ignoreEmptyAttributes`
so consumers can reuse it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kumilingus

packages/joint-core/types/dia.d.ts

@@ -612,9 +612,11 @@ export namespace Cell {
         mergeArrays?: boolean;
     }
 
+    type IgnoreEmptyAttributesCallback = (key: string, path: string[]) => boolean;
+
     interface ExportOptions {
         ignoreDefaults?: boolean | string[];
-        ignoreEmptyAttributes?: boolean | ((key: string, path: string[]) => boolean);
+        ignoreEmptyAttributes?: boolean | IgnoreEmptyAttributesCallback;
     }
 
     type UnsetCallback<V> = (