Replace ember-template-imports with content-tag

[gitKrystan]

Resources

https://github.com/embroider-build/content-tag
https://github.com/ember-template-imports/ember-template-imports/releases/tag/v4.0.0

Needs investigation

• We might not be able to maintain compatibility with ember-template-imports preprocessed code, which would mean we'd have to drop support for it altogether.

  This would cause compatibility issues with eslint-plugin-ember until they are using content-tag. Additionally, if they start using content-tag, running prettier via eslint-plugin-ember won't be possible until this library supports content-tag. (Use custom parser for gts/gjs ember-cli/eslint-plugin-ember#1942 will resolve this risk and make compat with ember-template-imports unnecessary.)

I've started a PR here (#153) with the processor switched, but all the tests are failing. Contributions welcome!