implement sync formdata parser

[KhafraDev]

Fixes #2890

Based on https://andreubotella.github.io/multipart-form-data/

import { Request } from 'undici'

const request = new Request('http://localhost', {
  method: 'POST',
  headers: {
    'Content-Type': 'multipart/form-data; boundary=----formdata-undici-0.6204674738279623'
  },
  body:
    '------formdata-undici-0.6204674738279623\r\n' +
    'Content-Disposition: form-data; name="fiŝo"\r\n' +
    '\r\n' +
    'value1\r\n' +
    '------formdata-undici-0.6204674738279623--'
})

const fd = await request.formData()
// > FormData { [Symbol(state)]: [ { name: 'fiŝo', value: 'value1' } ] }