Summary

• add parsing and display support for the SQL/JSONPath 2023 conversion methods
• implement numeric, boolean, string, and ISO datetime conversions
• preserve datetime item types for .type(), .string(), and comparisons
• document the new methods in README and add focused tests

Testing

• cargo test --all-features --lib --test pg_jsonpath --test sql_jsonpath_2023
• cargo clippy --lib --bins --all-features -- -D warnings
• cargo clippy --test sql_jsonpath_2023 --all-features -- -D warnings

Closes #2