Limit the number of concurrently open file watchers on macos (#73741)

### What?

This adds a workaround for a Node.js bug on MacOS which causes closing file watchers to be really slow:
nodejs/node#29949

It was fixed before here: #51826
But accidentically removed here: c5a8e09#diff-0ff576bd02e76f96680accff48ecbe4126a7f6bc4eafa547b6d44dee49bab770L88

packages/next/src/cli/next-dev.ts

@@ -291,6 +291,11 @@ const nextDev = async (
             ? startServerOptions.selfSignedCertificate.rootCA
             : defaultEnv.NODE_EXTRA_CA_CERTS,
           NODE_OPTIONS: formatNodeOptions(nodeOptions),
+          // There is a node.js bug on MacOS which causes closing file watchers to be really slow.
+          // This limits the number of watchers to mitigate the issue.
+          // https://github.com/nodejs/node/issues/29949
+          WATCHPACK_WATCHER_LIMIT:
+            os.platform() === 'darwin' ? '20' : undefined,
         },
       })