Deprecate and remove std/mime/multipart

We have FormData that has all the same functionality as std/mime/multipart as far as I can tell.

Let’s deprecate this module for 1.18, and remove it for 1.19.

[lucacasonato]

You call Request.form on the incoming request. This will give you a Promise<FormData>

[lucacasonato]

@ry How do you write multipart data using FormData?

const form = new FormData();
form.set("foo", "bar");
const req = new Request("https://google.com", { body: form });