Provide better gjs/gts config support for eslint 9

[NullVoxPopuli]

Resolves: #2190
Resolves: #2171 (sorta)

Unblocks: ember-cli/ember-cli#10533

[NullVoxPopuli]

new public import, only available via import, not require

[patricklx]

any chance to add a test?

[NullVoxPopuli]

any chance to add a test?

I think we'd have to add a whole acceptance suite -- which I'm a fan of -- and we used to have, but I think @bmish removed (because acceptance tests are slow).

I think it's valuable enough where we should just do it, and pay the cost of acceptance tests.

Is everyone ok with the tradeoff?
It would be great to not have to have the slow feedback loop with ember-cli's blueprint system to test changes to the configs in this project.

[NullVoxPopuli]

Gonna merge -- acceptance testing infra is too big to lump in to this PR and has some decisions that may need discussing / education / alignment / etc / whatever