add an option for a file edit path prefix for docker environments

[joachim-n]

The Ignition factory method applicationPath() is purely cosmetic: if a file  / var / www / html / web / modules / custom / joachim_debug / src / Controller / JoachimDebugController.php is shown in the error output, the filename is printed as modules / custom / joachim_debug / src / Controller / JoachimDebugController.php instead.

The URL for editing the file in an IDE remains the same.

However, if a PHP project is being run in Docker (for example using https://ddev.com/), the edit URL is no good, because it's an absolute path inside the Docker volume.

To fix this, there should be an extra Ignition factory method to set the file prefix. File edit URLs would then replace the applicationPath() with this.

So for example:

    Ignition::make()
      ->applicationPath('/var/www/html/web')
      ->editorPrefix('/home/joachim/Sites/myproject/web')

would then mean that the file

/ var / www / html / web / modules / custom / joachim_debug / src / Controller / JoachimDebugController.php

would link as

 / home/joachim/Sites/myproject/web / modules / custom / joachim_debug / src / Controller / JoachimDebugController.php

and my IDE would be able to open it.