Separate testnets and mainnets in aggregated chain list(s)

[geoknee]

https://docs.google.com/document/d/1lbNN_p8wuk3EhC1erlI3P3-dh7lx4UzWxA5Egf2Xugo/edit#heading=h.reolvji23szx

Blocked on a decision about exactly how we should split them out.

For example:

1. same file, but all mainnets come first and then all testnets
2. same file, ordered as (1) but nested under a "testnets"/"mainnets" key
3. a separate file for each list

[tessr]

I propose Option 3: Split chainlist into two separate files, one for mainnets and one for testnets. I think consumers of these lists would want to just see mainnets, or just testnets. We can have mainnets.toml/mainnets.json and testnets.toml/testnets.json.