postinstall script takes 30s on node19+ due to keepalive

From node 19 (e.g. Node 20, Node 22 LTS), they set keepAlive by default to true. This causes a behaviour (bug?) https://github.com/nodejs/node/issues/47228 where the process also does not exit for 30 seconds.

This can be fixed with this patch

before:
node node_modules/pg-formatter/bin/postinstall-script.js 0,50s user 0,25s system 2% cpu 30,255 total

after:

node node_modules/pg-formatter/bin/postinstall-script.js 0,60s user 0,27s system 61% cpu 1,391 total

patch: (too lazy to make a PR)

```diff
diff --git a/bin/postinstall-script.cjs b/bin/postinstall-script.cjs
index 1049542..19e7db8 100644
--- a/bin/postinstall-script.cjs
+++ b/bin/postinstall-script.cjs
@@ -5,6 +5,9 @@ const zlib = require('node:zlib');
const rimraf = require('rimraf');
const tar = require('tar-fs');

+// skript takes 30s otherwise
+// https://github.com/nodejs/node/issues/47228 where the process also does not exit for 30 seconds.
+https.globalAgent.keepAlive = false;
const destination = path.resolve(__dirname, '../src/pg-formatter');
const secondaryDestination = path.resolve(__dirname, '../dist/pg-formatter');
const libraryUrl = `https://github.com/darold/pgFormatter/archive/refs/tags/v5.5.tar.gz`;
```

Start in bin/postinstall-script.cjs and review the postinstall command shown in the issue, along with the Node.js keepalive behavior it references. Apply the requested keepalive change and run node node_modules/pg-formatter/bin/postinstall-script.js on Node 19 or newer, verifying that it exits promptly instead of taking about 30 seconds.

Written by the indexing model from the issue text.