Deprecate @ember/string usage.

[lifeart]

Since ember-source does not rely on @ember/string anymore.

We found usages of @ember/string in
@ember-data/adapter, @ember-data/debug, @ember-data/model, @ember-data/serializer,
@ember-data/store, ember-data deps.

Code search: https://github.com/search?q=repo%3Aemberjs%2Fdata%20%22%40ember%2Fstring%22&type=code

It seems we could try to replace/omit it and completely deprecate @ember/string.

Related stuff: ember-cli/ember-resolver#932, ember-cli/ember-resolver#933

[runspired]

We do not use ember-source for it, we declare the peer-dependency.

There isn't a good library out there that replaces it (or ember-inflector) I've investigated that a few times. I've been considering having EmberData ship its own string utils since long-term we only need camelize for the REST url builder, underscore for the ActiveRecord url builder, and pluralize for all builders. All other string functions i think end up unused except potentially for debug assertions once the deprecation for type-matches-registry-name is complete.

Closing since this work is already tracked in #8460 and #8147 and the RFC for strict types.