Move @ember/test-waiters to dependencies

[gorner]

Resolves #992 by moving @ember/test-waiters from package.json devDependencies to dependencies.

Confirmed this was fixed in my Ember 5.8 app that also uses ember-sortable (which requires a peer dependency of "@ember/test-waiters": ">= 3.0.1",) as follows:

• Removed package-lock.json and node_modules/
• Re-ran npm install (which generated a new package-lock with @ember/test-waiters v4 for ember-sortable)
• Booted ember serve and saw the same error logged in 992
• Replaced the ember-attacher entry in my package.json with "ember-attacher": "gorner/ember-attacher#move-test-waiters-to-deps", (i.e. this branch)
• Ran ember serve again and saw everything was working as expected

A few notes:

• Although there is now a v4 of test-waiters, I've stuck with v3 as there are breaking changes that require dropping support for Ember <4.0, which would imply a major version bump to ember-attacher. This will probably be needed in the near future, but it doesn't appear to be needed to resolve the current issue.
• The ember-try run for ember-release (Ember 6.0) fails due to breaking changes, specifically the requirement to switch to component template co-location. I see there is a separate issue for this already (Component Templates Co-location and Typescript #978) so for now I am marking the ember-release run as allowedToFail: true, adding a separate scenario for Ember 5.12, and indicating in the README that the addon is not yet compatible with Ember 6+.
• In the issue I noted that it might make sense to use peerDependencies instead so app developers could specify the version of test-waiters that works for them. For now it doesn't really make a difference because this package only works with v3 of test-waiters anyway. It might make more sense once the Ember 6.x compatibility issues are cleared.