Add @ember/string as a peerDep

[kategengler]

@ember/string without the external package is deprecated in ember-source v4.10

[kategengler]

From @ef4 in Discord, on why a peerDep vs a dep:

@ember/string contains shared module state (there are cache-based optimizations), which is one factor in trying to ensure a single copy.

It's not wrong for some addons to decide they'd rather use it as a dependency than a peerDependency, but it invites duplication, and in the case of @ember/string you'll also duplicate the caches.