fix: broken download tests

Declaring `browserslist` in package.json exposed a bug in the
download.js tests that wasn't causing failures before (but arguably,
should): one can't use arrow functions to mock constructors because
calling `new` on them doesn't work.  See the NOTE under:

https://jestjs.io/docs/es6-class-mocks#-module-factory-function-must-return-a-function

src/data/redux/thunkActions/download.test.js

@@ -21,9 +21,9 @@ jest.mock('@zip.js/zip.js', () => {
       close: mockZipClose.mockImplementation(() => Promise.resolve(files)),
       files,
     })),
-    BlobWriter: () => mockBlobWriter,
-    TextReader: () => mockTextReader,
-    BlobReader: () => mockBlobReader,
+    BlobWriter: function () { return mockBlobWriter; },
+    TextReader: function () { return mockTextReader; },
+    BlobReader: function () { return mockBlobReader; },
   };
 });