Add scalafix configuration for organizing imports

[kciesielski]

This configuration enables the "organize imports" action for Metals. I haven't changed the default settings too much, except setting groupedImports to Merge. This creates groups of imports if they are from the same package.

[adamw]

I'm a bit surprised though that any configuration is needed for such an obvious thing ;)

[kciesielski]

Actually the organize imports action has a default configuration, but it doesn't merge types from the same package.
The groups (custom/scala/java) are actually in the default config as well, so I'm removing this and leaving only

 groupedImports = Merge