Add support for node ports in edge definitions in .dot files

[RolandKluge]

Currently, the parser for .dot files only supports port definitions in node statements, but not in edge statements.
The DOT language definition states that ports are also possible in edge definitions: https://www.graphviz.org/doc/info/lang.html.
In the following example, the line containing the edge statement cannot be parsed.

digraph graph {
    1:s [label="1", x=0, y=5];
    2 [label="2", x=0, y=4];
    1:s -> 2:n;
}