Add :SortScalaImports command

This command sorts the imports at the top of the file (defined as the the first line starting with `import` to the first non-`import` and non-whitespace line).

It furthermore puts them into 3 groups:

* Scala/Java imports
* Third party libraries
* First party imports (what is considered first party code is configurable).

More information is available under `:help SortScalaImports`.