TypeScript type definitions for Blob, File, FormData

[harmony7]

This PR adds TypeScript typings for Blob, File, and FormData so they can be used in Compute apps in TypeScript.

Now tsc is happy with all this:

  if (event.request.headers.get('content-type')?.startsWith('multipart/form-data')) {
    const reqFormData = await event.request.formData();
    reqFormData.forEach((value, key) => {
      console.log(key, value);
    });
  }

  const file = new File(['hello, world'], 'temp.txt', {
    type: 'text/plain'
  });

  const formData = new FormData();
  formData.set('foo', '0');
  formData.set('bar', 'doo');
  formData.set('baz', file);

  const resp = await fetch(
    'https://httpbin.org/anything', {
      method: 'POST',
      body: formData,
      backend: 'httpbin',
    },
  );