Move to Kicad 6 support

[waffle87]

Sorry for super large PR — can be split up if that's preferred.
New format is essentially

.
├── corne-*.kicad_pcb	# general project file 
├── corne-*.kicad_prl	# local settings (removed in .gitignore)
├── corne-*.kicad_pro	# pcb data
├── corne-*.kicad_sch	# schematic data
├── fp-lib-table		# footprint library data
└── sym-lib-table		# symbol library data

(Addresses #147)

[foostan]

Thank you for your contribution, and I'm sorry super late. I'll migrate kicad7 after merging this PR.