Move formatter plugin to module JsonFormatter.Plugin

[wingyplus]

From discussion in https://elixirforum.com/t/json-formatter-a-json-code-formatter-for-mix-format/45030/2. It would be good to move all code in JsonFormatter module into JsonFormatter.Plugin module. The JsonFormatter will be use for documentation for now.

[RishiKumarRay]

I didn't got it, what we have to do here?

[wingyplus]

@RishiKumarRay technically, just rename module to JsonFormatter.Plugin