fix: timezone & offset issues

[hendurhance]

This pull request introduces several improvements and optimizations to the Chronos TypeScript date/time library, with a focus on more accurate timezone handling, improved parsing logic, and performance optimizations. The most significant changes are in how dates are parsed and compared in the presence of timezones, as well as the introduction of a formatter cache to reduce overhead in timezone calculations. Minor code cleanups and comment removals were also made for clarity.

Timezone and Parsing Improvements

• Enhanced parsing of offset-less ISO 8601 strings in Chronos, ensuring they are resolved in the correct timezone and rejecting invalid dates more robustly. This prevents silent fallback to the process timezone and ensures accurate wall-clock interpretation.
• All date comparison and arithmetic methods (isBefore, isAfter, isSame, isBetween, diff, etc.) now consistently use the instance's timezone for both operands, ensuring correct results when timezones are involved. This includes a new unitStart helper and logic for calendar unit differences. [1] [2] [3] [4] [5] [6]

Performance Optimizations

• Introduced a cache for Intl.DateTimeFormat instances in timezone.ts, significantly reducing the performance cost of repeated zone-aware date formatting and offset calculations. [1] [2] [3]

Code Cleanup and Maintenance

• Removed redundant comments and streamlined code in ChronosPeriod and ChronosPeriodCollection for better readability and maintainability. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Package and Export Adjustments

• Bumped the package version to 2.0.5 and removed the postpublish script from package.json. [1] [2]
• Minor export cleanups in index.ts and locales/index.ts. [1] [2]