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ARROW-8882: [C#] Add .editorconfig to C# code

Adding an `.editorconfig` using the rules from dotnet/runtime repo. See:

* https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/coding-style.md
* https://github.com/dotnet/runtime/blob/master/.editorconfig

Violating these rules won't fail the build, but Visual Studio will respect these rules when formatting the code and make suggestions.

The bulk of the violations were `var` usages. There was one unnecessary usage of `this.`. And the rest were naming static fields with `s_`. In `BitUtility.cs`, I made a slight optimization using the pattern recognized by https://github.com/dotnet/roslyn/pull/24621 instead of prefixing the fields with `s_`.

Tagging anyone who has contributed to the C# library (please let me know if I missed anyone):
@chutchinson @zgramana @nhustler @HashidaTKS @abbotware @pgovind @stephentoub

Closes #7246 from eerhardt/Editorconfig

Authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>