Add official PostHog plugin

[fercgomes]

Adds @analytics/posthog as an official in-repo plugin with browser and Node.js support.

What's included

• Browser: Loads PostHog via the official JS snippet, maps page, track, identify, and reset to PostHog's
  API. Exposes PostHog-specific features (feature flags, group analytics, privacy controls) as custom methods.
• Node.js: Uses posthog-node SDK with capture, identify, groupIdentify, and alias support. Includes
  flush and shutdown methods.
• PostHog's auto-pageview (capture_pageview) is disabled by default so the analytics library owns page tracking via
  analytics.page() without duplicates.
• Autocapture (clicks, inputs, DOM interactions) is enabled by default and works alongside the analytics library
  without conflicts.