feat(error-tracking): add PHP error tracking support

[cat-ph]

Summary

Add PHP error tracking support to posthog-php.

This includes:

• manual captureException(...) support for Throwable|string
• opt-in automatic capture for uncaught exceptions, PHP errors, and fatal shutdown errors

Changes

• add manual exception capture and $exception_list payload building
• add opt-in auto registration for exception, error, and shutdown handlers
• preserve default crash behavior when no prior exception handler exists
• preserve first installed client as the owner of global auto-capture handlers
• avoid double capture when warnings are promoted to ErrorException
• capture E_USER_ERROR through the error handler path as well

Test Plan

• php -l on changed files
• ./vendor/bin/phpunit

Full PHPUnit passes with the repo's existing non-fatal PHPUnit issues:

• no coverage driver
• existing warnings/deprecations

[hpouillot]

Few comments, but looks good to my rusty PHP eyes 👴🏻

[hpouillot]

:chef-kiss: