Add `useNuxtApp` and `defineNuxtPlugin` composables

Editor assessment

This feat subject is concrete: it names both composables being introduced and the bridge area that hosts them, so the header communicates the change on its own. The feat(bridge) classification is accurate, the imperative mood is observed, and the length is reasonable. No body follows to explain what `useNuxtApp` and `defineNuxtPlugin` unlock for module authors — the one gap in an otherwise complete message — and no footer is needed since nothing breaks. Clear, correct, and only marginally short of teaching depth; a good example.