Themes should be included from fragments

[zadjii-msft]

Basically the title. Just like schemes. Users should be able to

curl raw.github.com/microsoft/terminal/samples/accent.json > "%localappdata%\Microsoft\Windows Terminal\Fragments\themes\"

and have it just work 😉

[g0t4]

It would be nice to split up any configuration into a separate json file... why limit it to subsections! Just add to the merge strategy as-is.