Allow extension to listen on unix socket or pipes for Xdebug connections

[zobo]

Some users prefer UNIX socket for connections.

Xdebug can be configured to connect to UNIX socket https://xdebug.org/docs/step_debug#client_host

node also supports this: https://nodejs.org/api/net.html#serverlistenoptions-callback

However there are some considerations how best implement this. Currently proxy support needs a TCP port, that we get from listening even of net.createServer(). I would also not just reuse port but rather go the way node is doing it. Use path for UNIX socket unless port is defined.

Ref #421

[runephilosof-abtion]

If the file already exists, it will display an error. The error text should be clearer and suggest deleting the file, if it is an unused/leftover socket file from a previous session.

[zobo]

Will implement file check in #921 closing this.