Add GDT and/or neon bulb

[skull-squadron]

Both similar-ish in their own ways to a spark gap (already available), but different.

[MatNieuw]

With GDT I assume you mean gas discharge tube. Can that not be emulated with two high-voltage zener diodes in series?

For neon bulbs, see #294. I have not progressed further yet on that. Ideas welcome.