Rewrite ember-fetch to a v2 addon

[StephanH90]

Description

The goal is to make ember-fetch a v2 addon, removing all the old fetch polyfill stuff while maintaining backwards compatibility. This MR is the first step in this process.

This MR basically:

• removes all the broccoli magic stuff
• uses native fetch
• exposes a setupFetchWaiter function which provides backwards compatibility in tests
• continues to export a fetch from ember-fetch for backwards compatibility (its just the native fetch)

Breaking

The only change that has to be made, if somebody wants to upgrade to v9 is that they have to import a setupFetchWaiter function which wraps the native fetch with an waitForPromise. This means that fetch will continue to work in tests as it has done in the past. Using the setupFetchWaiter would look something like this:

import { setupFetchWaiter } from 'ember-fetch';

setupFetchWaiter();

In development and production builds this does nothing.

Next steps

In the next steps various other addons will have to be pulled into test apps to make sure that they continue to work with this version of fetch.

[NullVoxPopuli]

Looks like nohe modules got committed 🙃

[NullVoxPopuli]

We'll want to keep README.md and RELEASE.md, .release-it.json, and the CHANGELOG.md should be move in to the ember-fetch directory.

[NullVoxPopuli]

we probably want to keep this as well

[NullVoxPopuli]

this fetch will be undefined unless isTesting, which isn't quite right.
We'll want it to be globalThis.fetch

[NullVoxPopuli]

Can you re-target main/master? I closed the next branch, and made a v8 for bugfixes/backports