Generates set.json files from Scryfall

I should have done this in the first place instead of figuring out how
to use MTGJSON files. Scryfall structures its data a bit differently and
notably includes a list of keywords for each card.

Having these set.json files enables easier analysis of the keyword
content of each set.

set_codes.csv

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+2ED
+3ED
+4ED
+5ED
+6ED
+7ED
+8ED
+9ED
+10E
+M10
+M11
+M12
+M13
+M14
+M15
+ORI
+M19
+M20
+M21
+POR
+P02
+PTK
+S99
+ARN
+ATQ
+LEG
+DRK
+FEM
+HML
+ICE
+ALL
+MIR
+VIS
+WTH
+TMP
+STH
+EXO
+USG
+ULG
+UDS
+MMQ
+NEM
+PCY
+INV
+PLS
+APC
+ODY
+TOR
+JUD
+ONS
+LGN
+SCG
+MRD
+DST
+5DN
+CHK
+BOK
+SOK
+RAV
+GPT
+DIS
+CSP
+TSP
+PLC
+FUT
+LRW
+MOR
+SHM
+EVE
+ALA
+CON_
+ARB
+ZEN
+WWK
+ROE
+SOM
+MBS
+NPH
+ISD
+DKA
+AVR
+RTR
+GTC
+DGM
+THS
+BNG
+JOU
+KTK
+FRF
+DTK
+BFZ
+OGW
+SOI
+EMN
+KLD
+AER
+AKH
+HOU
+XLN
+RIX
+DOM
+GRN
+RNA
+WAR
+ELD
+THB
+IKO
+ZNR
+MMA
+MM2
+EMA
+MM3
+IMA
+A25
+UMA
+2XM
+MH1
+CMR
+JMP