Prepare for V2 Addon conversion.

[NullVoxPopuli]

Note the diff stats are way more intimudating than this PR actually is.
It's 90% "moves" + some boilerplate.
Very little has actually changed.

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The primary purpose of V2 Addon conversions is to get ember apps installing deps, booting and building faster
(for me, anyway)

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qunit-dom has never been compatible with modern packagers -- this is due to the issue reported in #2022 partially due to incorrect package.json config.

Since we want to also be good stewards of the ember ecosystem, we don't want to just fix the issue for non-ember users. We want to make sure that ember users don't pay the v1 addon / broccoli infrastructure cost when consuming this library.

Step one: convert to monorepo

• extracting the tests to their own project
• ensuring CI is up to date to be ok with the new structure
  Step two: convert the addon to v2
• thankfully the library is already most of the way there -- this step will mostly be package.json tweaks
  • the package cross-compiles to IIFE (I presume for non-ESM CDN usage?) and ESM already.
• this step will mostly be getting rid of the index.js, and other "emberisms"

For additional context on this process, I've written up this guide: https://gist.github.com/NullVoxPopuli/eafc7dad6547de5e730098498b829e1f

• I've skipped the 1. Convert solo-repo to single-package monorepo step because I know that the MainMatter folks are more up to date with what's going on in the ember ecosystem than most, and the step is very small anyway.

Thankfully, qunit-dom is already using pnpm 🎉

[BobrImperator]

Looks fine.
Some of the comments I added seem like they can be ignored since I made them before seeing #2028.

For consistency across our other repos, I'd like to ask to put each of the packages to /packages/ directory.
Other than that everything looks perfect 👍

[BobrImperator]

Here the working-directory makes sense to me, so I'd not change that.

Note; Although on another note we should consider setting up scripts for that in the root package.json.

[NullVoxPopuli]

note, I will be discarding all package.json / lockfile changes because having a chain of 3 PRs to manage over a long period of time while upstream is changing so much is hard. (normally this wouldn't be the case, but the repo is getting restructured)

However, This PR: #2031
should be reviewed and merged first, as it's important for a changelog entry to happen before the v2 addon stuff.

[NullVoxPopuli]

Also, I'm going to start squashing all my commits in each PR.
The entirety of #2028 is now in conflict (every file) (this is just the nature of re-structuring the repo upstream of changes 🤷 )
Though, it appears that the conflicts traverse through every commit, so...

I'm going to undo moving everything in to packages/* and we can change that after the PRs are merged.

This allowed me to be able to rebase #2028