tweak XLSX export worker to make Piscina happy under Electon (#646)

Adds a dummy default export to the worker exporter script used
for producing XLSX. This method exists only to make Piscina
happier. With it, Piscina will load this file using a regular
require(), which under Electron will deal fine with Electron's
ASAR app bundle. Without it, Piscina will try fancier methods
that aren't at the time of writing correctly patched to
deal with an ASAR app bundle, and so report that this
file doesn't exist instead of exporting an XLSX file.

I tried various other solutions such as upgrading Electron,
unpacking various files, patching Piscina, and this was
overall the simplest.

See https://github.com/gristlabs/grist-electron/issues/9

app/server/lib/workerExporter.ts

@@ -224,3 +224,14 @@ export function sanitizeWorksheetName(tableName: string): string {
     .replace(/^['\s]+/, '')
     .replace(/['\s]+$/, '');
 }
+
+// This method exists only to make Piscina happier. With it,
+// Piscina will load this file using a regular require(),
+// which under Electron will deal fine with Electron's ASAR
+// app bundle. Without it, Piscina will try fancier methods
+// that aren't at the time of writing correctly patched to
+// deal with an ASAR app bundle, and so report that this
+// file doesn't exist instead of exporting an XLSX file.
+//   https://github.com/gristlabs/grist-electron/issues/9
+export default function doNothing() {
+}