fix(cosmic-swingset): re-enable export of stringify

dapp-svelte-wallet does a deep-import of repl.js, so it's really a part of
the cosmic-swingset external API surface. The function signature of
`stringify()` has changed, but that particular caller isn't affected.

packages/cosmic-swingset/lib/ag-solo/vats/repl.js

@@ -6,7 +6,12 @@ import { Nat } from '@agoric/nat';
 import makeUIAgentMakers from './ui-agent';
 
 // A REPL-specific JSON stringify.
-function stringify(value, spaces, inProgress = new WeakSet(), depth = 0) {
+export function stringify(
+  value,
+  spaces,
+  inProgress = new WeakSet(),
+  depth = 0,
+) {
   if (Object(value) !== value) {
     if (typeof value === 'bigint') {
       return `${value}n`;