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

[eerhardt]

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 dotnet/roslyn#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

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8882

[pgovind]

LGTM

[zgramana]

This is helpful, thanks @eerhardt! Editorconfig is preferable for those of us using VS Code on *nix.

[eerhardt]

Also tagging @mr-smidge (sorry I forgot initially)

[emkornfield]

If there is a mechanism to fail the build with these rules I would recommend trying to add it to the list for check to avoid regressions

[eerhardt]

If there is a mechanism to fail the build with these rules I would recommend trying to add it to the list for check to avoid regressions

There is an experimental package that hasn't been released yet that will allow for enforcement during the build:

https://dotnet.myget.org/feed/roslyn/package/nuget/Microsoft.CodeAnalysis.CSharp.CodeStyle

When this feature is shipped, we can update to using it to start enforcement of code-style rules.