feat(interpreter): add ROUND, MOD, MIN/MAX, TIME, and date part built…

[kas2804]

Closes #4

Overview
This PR adds the missing standard dBASE III stateless built-in functions to the expression engine, as outlined in the issue.

Changes Included:

1. Math Functions: Added ROUND(), MOD(), MAX(), and MIN() to Builtins.ts.
2. Date/Time Functions: Added TIME(), YEAR(), MONTH(), and DAY() to Builtins.ts.
3. Testing: Added corresponding test cases for all new functions in Builtins.test.ts to ensure correct behavior and edge-case handling (e.g., negative numbers, invalid dates).
4. Documentation: Updated the README.md command tables

Testing Performed:
All tests passed successfully locally via vitest.

[DDecoene]

Thank you so much for this, @kas2804! 🙏 This is exactly the kind of contribution that makes the project better — eight new built-ins (ROUND, MOD, MAX, MIN, TIME, YEAR, MONTH, DAY), each with a clean implementation following the existing n()/s() helper conventions and a matching test. ROUND's default-decimals handling and the guarded date parsing were nice touches.

Your work brings the W3Script expression engine noticeably closer to full dBASE III parity. I've credited you in the README's new Contributors section, the CHANGELOG, and CLAUDE.md as part of the v0.8.0 release follow-up (#18). Looking forward to more — thank you for helping keep the dBASE III spirit alive! 🙌