Bison switching to .gv Graphviz output

[magjac]

@smattr

I did not realize .gv was preferred over .dot. Maybe these are widely known to others.

I was aware of this, but I still use .dot because I like Graphviz and the DOT language, but I don’t like Microsoft or Windows.

Also discussed in https://forum.graphviz.org/t/what-is-the-file-extension-for-dot-file/3