feat: support render function for json directory listings

[lx3133584]

Problem

When list.format is set to json (or default JSON mode), configuring a custom list.render function was ignored for JSON responses and only invoked when querying with ?format=html.

Root Cause

dirList.send handled list.render exclusively inside the format === 'html' branch, skipping user-defined custom directory rendering for JSON output.

Fix

• Updated dirList.send to invoke options.render(dirs, files, format) for JSON listings when list.render is provided.
• Added ListJsonRender and ListFormat to TypeScript declarations and extended test definitions in types/index.tst.ts.
• Added documentation and examples for JSON rendering in README.md.

Testing

• Added unit tests in test/dir-list.test.js verifying custom JSON rendering and format argument propagation.
• Verified all 348 unit tests pass with 100% line, statement, and branch coverage, and typecheck passes cleanly.