Move editor services command module and relevant public types into their own assembly

[rjmholt]

Summary of the new feature

PSES uses a fresh ALC to hide its dependencies from PowerShell and other modules. This makes it hard to use those things that are meant to be exposed. Even though the PSES assembly is loaded into default ALC, PowerShell doesn't quite see it that way.

The simplest fix for this is to move it all out into a new assembly. This will break some of the type names from a fully qualified perspective ([Microsoft.PowerShell.EditorServices.Extensions.EditorFile, Microsoft.PowerShell.EditorServices] will become something like [Microsoft.PowerShell.EditorServices.Extensions.EditorFile, Microsoft.PowerShell.EditorServices.Extensions] or something like that). However, this will mean that PowerShell can see these types with unqualified names.

We'll work with extension authors to minimise the impact of any breaking changes here.