Maintenance, V2, modern `fetch`, etc

[NullVoxPopuli]

I've created a v8 branch that we can use for bugfixes against the current major if we need it.

The main / master branch will be used for implementing the modernizations

Some high-level things that need to happen

• have a single migration path with backwards (or co-mingled) compatibility between old and new, as it'll take some time for a major release to propagate the ecosystem
  • this may require a monorepo setup so we can configure a app + addon combo that uses old ember-fetch with the new one (and test that they work without stomping on each other)
    • first, we'd want to convert to a single-package monorepo
    • probably switch to pnpm
• we need to import from "ember-fetch", not "fetch"
• we need to remove the build time code
• update infra / ci / etc
• probably switch to pnpm
• convert to v2-addon (native package)
  • this would allow v2 addons to use ember-fetch
• retain the test/waiter wrapping around fetch
  • we don't want to add this to @ember/test-waiters by default, because that would break folks' apps who aren't using ember-fetch but are using native fetch. The waiting-capabilities around ember-fetch that we have today should be opt in, as they are today
    test-waiters could still provide the ultility to import to make fetch behave like the ember-fetch implementation does today -- and then ember-fetch can import that utility and wrap fetch with an export so that folks can import an instrumented fetch (behaving similar to what the existing import does)
• drop support for node < 18 (so we can use native fetch)
• drop support for IE11 (which doesn't inherently affect ember < 4, so 3.x can be supported with this one caveat)

Be sure to read through

• Archive this repo? #656