Feature: #79343 - Allow overriding PATH_site via environment variable 

See forge#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.