Replace the is-plain-obj dependency

It does some checks that we don’t need. Just checking the prototype is
sufficient.

package.json

@@ -29,8 +29,7 @@
     "test": "c8 node --enable-source-maps dist/test.js"
   },
   "dependencies": {
-    "@types/estree": "^1.0.0",
-    "is-plain-obj": "^4.0.0"
+    "@types/estree": "^1.0.0"
   },
   "devDependencies": {
     "@types/node": "^20.0.0",

src/estree-util-value-to-estree.ts

@@ -6,7 +6,6 @@ import {
   type SimpleLiteral,
   type VariableDeclarator
 } from 'estree'
-import isPlainObject from 'is-plain-obj'
 
 /**
  * Create an ESTree identifier node for a given name.
@@ -364,12 +363,15 @@ export function valueToEstree(value: unknown, options: Options = {}): Expression
       for (const entry of val) {
         analyze(entry)
       }
-    } else if (options.instanceAsObject || isPlainObject(val)) {
+    } else {
+      const proto = Object.getPrototypeOf(val)
+      if (proto != null && proto !== Object.prototype && !options.instanceAsObject) {
+        throw new TypeError(`Unsupported value: ${val}`, { cause: val })
+      }
+
       for (const key of Reflect.ownKeys(val)) {
         analyze((val as Record<string | symbol, unknown>)[key])
       }
-    } else {
-      throw new TypeError(`Unsupported value: ${val}`, { cause: val })
     }
     stack.pop()
   }