perf: faster object property and date/time serialization

[mcollina]

• Fold commas (and the leading { when a required property comes first) into the property key literals, so each property is serialized with a single string append instead of two or three.
• Format date-time, date and time manually instead of via toISOString and intermediate Date allocations (~4.5x, ~16x and ~17x respectively). Expanded years and invalid dates fall back to the previous path.
• Replace the bound asInteger with a closure.

Benchmarks (Node 24): formatted time +1605%, formatted date +1493%, date-time +355%, objects +7-15%, arrays of objects +15%. No regressions.

Also fixes invalid JSON output ({,"b":2}) when required lists a key not declared in properties and the first declared property is undefined; regression test added.

[Tony133]

there are some conflicts in the index.js

[nigrosimone]

Perhaps the readme.md needs to be updated as well https://github.com/fastify/fast-json-stringify#benchmarks