Pull router_js into the Ember monorepo

[mansona]

With the Router RFC coming very soon we need to have router_js in the main monorepo. This PR does that work (with preserving history)

Still todo:

• make sure the tests are actually running in CI
• remove unnecessary files from router_js
• make sure that only the entrypoint is added to renamed packages in package.json
• profit 🎉

For anyone wanting to review this PR and not wanting to read all history from all of router_js then you can start at this commit and use the "Next commit" links at the top to go forward

Alternatively you can see the full diff with actual changes by looking at this link: mansona/ember.js@60942e2...mansona:ember.js:router-js

sus browserstack result

[kategengler]

The Browserstack errors are consistent

[mansona]

The Browserstack errors are consistent

This is very strange 🙈 the CI run just before I removed an eslint commit passed without any issue: https://github.com/emberjs/ember.js/actions/runs/23306124516 so something strange/flakey is going on here

And the errors have nothing to do with the router https://github.com/emberjs/ember.js/actions/runs/23309427423/job/67814938742#step:6:128 the last two runs failed on not ok 17 Safari 15.6 - [1013 ms] - Strict Mode - Runtime Template Compiler (explicit) - built ins: Can use hash

It could be that integrating the router_js test suite has introduced new flakiness but there is 0% chance that there was a causal reason why this is failing (and only on safari too 🫠 )

[mansona]

@kategengler @NullVoxPopuli I ended up removing the custom backburner wrapper in the tests since the flakeyness was probably being caused by that leaking (somehow)

To review that last commit you should probably ignore whitespace 🙈 the change is much smaller when you do because of indentation changes and prettier 🫠

[NullVoxPopuli]

test changes seem reasonable!