fix(useImport): 修正imgExporting赋值错误并添加getImgExporting方法

修复将imgExporting错误赋值给importJSON的问题，并添加getImgExporting方法以获取当前imgExporting状态

src/hooks/useImport.ts

@@ -227,7 +227,8 @@ export default () => {
     const win = window as any
     if (!win.exportJSON) win.exportJSON = exportJSON2
     if (!win.readJSON) win.readJSON = readJSON
-    if (!win.importJSON) win.imgExporting = imgExporting.value
+    if (!win.imgExporting) win.imgExporting = imgExporting.value
+    if (!win.getImgExporting) win.getImgExporting = () => imgExporting.value
   }
 
   // 导入pptist文件