Support hidden and deleted pins
As discussed at https://github.com/pointhi/kicad-footprint-generator/pull/586 and specifically at https://github.com/pointhi/kicad-footprint-generator/pull/586#issuecomment-680057204, I've added support for deleted pins (pins which are not present but do not consume a pin number) and custom pad layouts like SOT23.
Here are some example footprints (from test_hidden_deleted_pins.yaml):
I had to change scripts/Packages/Package_Gullwing__QFP_SOIC_SO/size_definitions/soic.yaml just to conform to this new terminology.
@pointhi The node PadArray seems to barely resemble the original implementation. There are so many caveats with deleted and hidden pins now. Since the values were being validated already, I chose to use a negative number as a filter since negative pin numbers seemed like they should not be allowed. Are you OK with this or do you have some other direction you'd like to pursue?