fix(prefer-promises/fs): cover fs.watch

[lprnmns]

Problem

n/prefer-promises/fs does not report fs.watch() callback usage, even though Node.js provides fs.promises.watch(). The rule documentation says callback APIs should be disallowed in favor of promise APIs, so this leaves one supported API unprotected.

Fix

Add watch to the rule’s existing fs call trace map. This covers CommonJS, ESM, and process.getBuiltinModule("fs") references through the shared tracing logic, with no API or dependency changes.

Tests

• npm run test:mocha -- tests/lib/rules/prefer-promises/fs.js — passed (56 passing)
• npm test — passed (3149 passing, 3 pending)
• git diff --check — passed

Compatibility

This adds a diagnostic only when the opt-in n/prefer-promises/fs rule is enabled. The change does not alter runtime APIs, generated files, or dependencies.

Related issue

Independent reproduction; no existing issue or pull request for fs.watch was found in the final duplicate search.

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