Feature: #79343 - Allow overriding PATH_site via environment variable

See Issue #79343

Description

It is now possible to define the PATH_site constant, which acts as a basis for any entry point running a TYPO3 system, via the environment variable TYPO3_PATH_ROOT.

This variable is automatically calculated and set for any TYPO3 installation set up via composer, making it possible to run the TYPO3 command line interface from any location of the system.

Impact

When using the command line entry-point typo3/sysext/core/bin/typo3 with composer, it can also be called from a projects' default bin/ directory.