fix: credit package.json prettier string configs

[V1RE]

What

Credit package.json "prettier": "<package>" string configs as referenced dependencies in the Prettier plugin.

Also adds an issue #818 fixture and regression integration test.

Why

Closes #818.

Prettier supports using the package.json "prettier" field as an external config package reference, for example:

{
  "prettier": "@scope/prettier-config"
}

Fallow previously treated that shared config package as an unused dev dependency because the Prettier plugin only handled object-style configs and plugins arrays.

Test plan

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace -- -D warnings