Add type restrictions to json

[Vici37]

trafficstars

This is the output of compiling cr-source-typer and running it with the below incantation:

CRYSTAL_PATH="./src" CRYSTAL_HAS_WRAPPER=1 ./typify spec/std_spec.cr
--error-trace --exclude src/crystal/ 
--stats --progress 
--union-size-threshold 2
--ignore-private-defs 
--ignore-protected-defs  
src/json

This is related to https://github.com/crystal-lang/crystal/pull/15682 .