[NWD] Format EPPlus with CSharpier. Also: - Add dotnet-csharpier and .editorconfig files - Clean up identifier naming - Misc cleanup Change-Id: I73e05e4ff47dd857323ec5da8aec4b3bf021f496 Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/207429 Reviewed-by: Hughes Hilton <hugheshilton@google.com>