Add bounded isEmptyDir checks

[Victor-Su-Ortiz]

Summary

Fixes #1037.

Add isEmptyDir (promise/callback) and isEmptyDirSync, exported through the CommonJS and ESM entry points. Both use opendir and read a single entry, then close the handle. Filesystem errors are preserved, including a read error if closing also fails.

Includes API documentation and tests for empty/nonempty directories, missing paths, files, symlinks, bounded reads, and cleanup.

Verification

• The public API regression tests fail on the original exports and pass with the change.
• Focused suite: 15 passing.
• npm test: lint, 760 CommonJS tests, and ESM tests pass; 8 existing tests pending. Cross-device tests were not enabled.
• git diff --check: passed.

Verified locally on macOS; other platforms remain for CI.

AI assistance

Implementation and initial tests were produced through OpenCode using opencode/space-bunny-free (Space Bunny). Codex reviewed the diff and independently ran the checks above. This draft has not received human code review.

[RyanZim]

Please undraft once you've reviewed this yourself and this PR is ready for review. From a quick cursory look, this PR could use some cleanup.

Going forward, I'd prefer if PRs were opened only after you've given the code a human review. But thanks for being transparent and upfront about the AI usage and status of the PR.