Updating to Remix v2

[rissois]

Trying to make RemixGCF compatible with Remix V2.

I'm a little out of my element here, so this PR is untested and should not be merged. I wanted to get the ball rolling and see if anyone can help me out with one snag in the return headers line of createRemixHeaders:

Type 'import("/Users/XXX/Desktop/remix-google-cloud-functions/node_modules/undici-types/fetch").Headers' is not assignable to type 'Headers'.
  The types returned by '[Symbol.iterator]()' are incompatible between these types.
    Property '[Symbol.iterator]' is missing in type 'SpecIterator<[string, string], any, undefined>' but required in type 'IterableIterator<[string, string]>'.ts(2322)
lib.es2015.iterable.d.ts(53, 5): '[Symbol.iterator]' is declared here.

I could roll down the Remix to v2.8.1, which is prior to the introduction of undici, unless someone can explain how to fix this error.