FileIO compatibility

[denizyuret]

Would it be possible to make JSON compatible with FileIO like JLD or JLD2? Right now I can save/load model files in different formats relying on FileIO figuring out the format looking at file extension and/or magic numbers and would like to do the same with JSON.

[KristofferC]

I really like how lean and non-bloated JSON.jl is. FileIO has had problems with increasing load times (JuliaPlots/Plots.jl#1379) and also does a lot of questionable stuff like evaluating methods into other packages (https://github.com/JuliaIO/FileIO.jl/blob/a676a5c51fbc0181ebe46348c63cc6de9e09bcae/src/loadsave.jl#L197-L219) and other things that are not really covered by the stability guarantees. JSON.jl is a very important package since e.g. the benchmarking stack uses it which is regularly run on Nightly, so keeping it as stable as possible is useful. There can always be a JSonFileIO package for those that like using FileIO.

[RomeoV]

Maybe it's worth revisiting this with package extensions?
JSON is one of the few formats I've run into that don't support FileIO yet.
EDIT: Seems like e.g. TOML also doesn't support it... Is there a reason for these serialization formats to not be supported that I'm missing?

[RomeoV]

Seems like there's also #366.