Fix functions in preset settings

Closes GH-77.

lib/configuration.js

@@ -84,7 +84,7 @@
 import assert from 'node:assert/strict'
 import path from 'node:path'
 import {fileURLToPath, pathToFileURL} from 'node:url'
-import structuredClone from '@ungap/structured-clone'
+import extend from 'extend'
 import createDebug from 'debug'
 import isPlainObj from 'is-plain-obj'
 import {resolvePlugin} from 'load-plugin'
@@ -349,7 +349,7 @@ async function merge(target, raw, options) {
       )
     }
 
-    target.settings = structuredClone({...target.settings, ...result.settings})
+    target.settings = extend(true, target.settings, result.settings)
   }
 
   /**
@@ -494,7 +494,7 @@ async function merge(target, raw, options) {
  */
 function reconfigure(entry, value) {
   if (isPlainObj(entry[1]) && isPlainObj(value)) {
-    value = structuredClone({...entry[1], ...value})
+    value = extend(true, entry[1], value)
   }
 
   entry[1] = value

package.json

@@ -34,9 +34,9 @@
     "@types/is-empty": "^1.0.0",
     "@types/node": "^20.0.0",
     "@types/unist": "^3.0.0",
-    "@ungap/structured-clone": "^1.0.0",
     "concat-stream": "^2.0.0",
     "debug": "^4.0.0",
+    "extend": "^3.0.0",
     "glob": "^10.0.0",
     "ignore": "^5.0.0",
     "is-empty": "^1.0.0",
@@ -52,8 +52,8 @@
     "yaml": "^2.0.0"
   },
   "devDependencies": {
+    "@types/extend": "^3.0.0",
     "@types/parse-json": "^4.0.0",
-    "@types/ungap__structured-clone": "^1.0.0",
     "c8": "^9.0.0",
     "prettier": "^3.0.0",
     "remark": "^15.0.0",