Replace Affjax with Fetch

The fetch library by @sigma-andex and @i-am-the-slime takes advantage of Node's support of the fetch API since Node 18. It's a little more comfortable for sending buffer data in Node (see #642), and it has no JS library dependencies — Affjax relies on the elderly xhr2 library.

This issue tracks replacing our use of Affjax with Fetch. Fortunately we don't make many requests; they're all in the following modules:

App.Prelude, used in the withRetryRequest helper
App.Effect.Source, used to download tarballs from GitHub into a buffer
App.Effect.Storage, used to download tarballs from the registry backend into a buffer
App.Effect.Pursuit, used to POST json data to Pursuit to upload packages

We'll also want to drop the dependencies in spago.yaml:

registry-dev/app/spago.yaml

Lines 10 to 11 in 8177cd3

    
           - affjax 
        
           - affjax-node

As well as drop the xhr2 dependency from package.json:

registry-dev/app/package.json

Line 10 in 8177cd3

"xhr2": "^0.2.1",

I am volunteering to work on this: #642 (comment)