Support a 'port' attribute for launch configs of type 'launch' too

[weinand]

If a launch config tries to use an existing script (e.g. npm script) to launch a program for debugging, it might be difficult to pass a debug port number to that script, or the script might use a fixed port.

To accommodate for this, we should support to specify a port for 'launch' in the same way as for 'attach'.