Use File-Scoped Namespaces and Global Implicit Usings

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Refactors the entire codebase to use File-Scoped Namespaces and Global Implicit Usings. In addition the following namespaces have been added as optional implicit usings for package consumers.

• SixLabors.ImageSharp
• SixLabors.ImageSharp.PixelFormats
• SixLabors.ImageSharp.Processing

There are no functional changes, only organizational so I'm not expecting nor needing a review.

See the following blog posts for more information:

• https://blog.jetbrains.com/dotnet/2021/10/28/file-scoped-namespaces-in-csharp-10/
• https://blog.jetbrains.com/dotnet/2021/11/18/global-usings-in-csharp-10/