Added MAC Core format datetime helper

Property	Value
description	Added the preset-driven shared datetime formatting utility and its documented extension hooks.
version	1.0.0
type	Create
related	Format Datetime, Extensibility

• Added mac_core_format_datetime() as the shared preset-driven datetime helper in the public utility surface.
• Shipped the built-in event-oriented datetime preset and the core plain, HTML, timezone, diff, and attr output paths used by the helper.
• Added extension hooks for datetime config, presets, and views so child themes or site plugins can extend formatting without editing MAC Core.
• Kept FormatDatetime as the shared formatting layer for builders, templates, and light site code instead of scattering project-specific date logic across themes.