Implement `FormData`

[Electroid]

We should implement the FormData API.

const request = new Request(new FormData());
const response = new Response();
const form = await response.formData();

[Jarred-Sumner]

I agree. The reason we haven't is because WebKit's implementation is closely tied to HTMLFileElement. I don't know if we can just use WebKit's version.

[Electroid]

Closing as a duplicate, now being tracked here: #621