feat(dia.Element): add getPortGroupNames()

[kumilingus]

Description

Add a convenient method for those who want to iterate over all port groups.

element.getPortGroupNames().forEach(group => {
   const groupPortPositions = element.getPortsPositions(group);
  /* ... */ 
});

Documentation

element.getPortGroupNames()

Returns an array of port group names of the element. It returns all port group names defined on the element regardless of whether the port group has any ports or not.