chore: restrict prepare script to Node.js >= 20 (#681)

`vp config` in the `prepare` script requires Node.js >= 20, but the
package supports >= 18.19.0. This causes failures when installing on
Node.js 18.x.

## Changes

- **`package.json`**: Replace bare `vp config` with an inline Node.js
version guard that skips execution on Node.js < 20:

```json
"prepare": "node -e \"const v = parseInt(process.versions.node, 10); if (v >= 20) require('child_process').execSync('vp config', {stdio: 'inherit'});\""
```

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Author: Copilot

package.json

@@ -66,7 +66,7 @@
     "ci": "vp run cov && vp run prepublishOnly && npm pack && attw --pack",
     "clean": "rm -rf dist && tsc -b --clean",
     "prepublishOnly": "vp run build",
-    "prepare": "vp config",
+    "prepare": "node -e \"const v = parseInt(process.versions.node, 10); if (v >= 20) require('child_process').execSync('vp config', {stdio: 'inherit'});\"",
     "check": "vp check",
     "check:fix": "vp check --fix"
   },