Move usings to top of file and using file-scoped namespaces

[MattKotsenas]

This is a mechanical change to:

1. Move usings to the top of the file
2. Use file-scoped namespace declarations

These two refactorings are done together because they touch every file and would otherwise be a source of merge conflicts.

In order to enforce these changes going forward, I made the following changes to .editorconfig:

• dotnet_separate_import_directive_groups = false to not require the blank space between "1st party" and "3rd party" usings
• Upgrade csharp_using_directive_placement = outside_namespace:warning to a warning
• Add csharp_style_namespace_declarations = file_scoped:warning as a warning

[MattKotsenas]

Reviewer's note: Consider hitting "ignore whitespace" and use the split view.