Add support for SMD-only footprints, and for test pins.

[infovore]

Test pins are broken out as TP1-TP6; however, as per the datasheet, TP4/TP5 are ignored.

Test pins have through-hole straight through to the pad on the TH footprints, and just a 1.5mm square pad on the SMD only footprints.

The main reason to break out test pins is access to the USB DP/DM pins (if you want to break out a Pico to an external USB connector), and access to the BOOTSEL pin.

Pico_Test_Pins schematic symbol:

Pico_Test_Pins_SMD_TH footprint:

Pico_Test_Pins_SMD footprint, as sample of SMD-only footprints:

Footprints not tested in anger, so this is very much a proposal PR rather than a "sure merge away" PR.

[HeadBoffin]

Thanks for this. I'll run this through KiCad over the next couple of days so neither of us wrong foot anyone in the future and then merge.

[infovore]

Yeah, not a problem. Merging KiCad files is still always a bit painful!