Add dotnet_separate_import_directive_groups to .editorconfig

This is the default setting in VS, but folks who have changed the setting can cause noise in PRs unless we pin the setting at the repo level.

.editorconfig

@@ -40,6 +40,7 @@ indent_size = 4
 [*.{cs,vb}]
 # Sort using and Import directives with System.* appearing first
 dotnet_sort_system_directives_first = true
+dotnet_separate_import_directive_groups = false
 dotnet_style_qualification_for_field = true:warning
 dotnet_style_qualification_for_property = true:warning
 dotnet_style_qualification_for_method = true:warning