Replace chrono feature with jiff

[fabian-braun]

Summary

• replace the optional chrono feature/dependency with jiff
• port RFC3339 parse/format, Display, and conversion impls to jiff equivalents
• document the breaking feature change and update the accepted infinity formatting expectation

Fixes #147

[Copilot]

Pull request overview

This PR replaces the optional chrono-backed integration with a jiff-backed one, migrating RFC3339 parsing/formatting, Display, and conversion impls behind a new jiff feature flag, and updates expectations around infinity formatting.

Changes:

• Replace the optional chrono feature/dependency with jiff, including deleting src/chrono.rs and adding src/jiff.rs.
• Port Time RFC3339 parse/format and Display/conversion impls to jiff equivalents, including ±∞ formatting for out-of-range values.
• Update tests and changelog to reflect the feature rename and new formatting expectations.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/serde.rs	Switch RFC3339 serde helper gating from chrono to jiff.
src/lib.rs	Rename public feature module to jiff and adjust formatting-related tests.
src/jiff.rs	New jiff-based formatting/parsing/conversion implementations and Display impls.
src/chrono.rs	Removed chrono-based integration module.
CHANGELOG.md	Document the breaking feature rename (chrono → jiff).
Cargo.toml	Swap optional dependency/feature from chrono to jiff.
Cargo.lock	Update lockfile to remove chrono-related crates and add jiff-related crates.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.