monkey-patch formData to enable multipart parsing

[Rich-Harris]

This is an alternative to reverting #5117 (#5290). It's ugly, but we can use the multipart form data parsing in node-fetch by monkey-patching the undici Request object's formData method to construct a node-fetch Request with the raw data and call formData on that.

Hideous, but it works. If nodejs/undici#974 is resolved, we can remove this hack.

Marking as draft for now as it should really have a test.

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[benmccann]

Hmm. Looks like this might only work on Node 18 for some reason. Maybe the polyfill isn't working on Node 16 as expected?

[Rich-Harris]

Very weird that the allows return value to be a Response test fails in CI. It works locally using the same Node version

[Rich-Harris]

Ok, so it was failing in CI because for some reason I don't fully understand some polyfills were being applied and some weren't, which led to an instanceof check failing. I think the best solution is to always apply all polyfills consistently