Added .editorconfig, removed TestExtensions project

[satano]

.editorconfig

• Added minimal settings for file types used in project.
• Turn off obsolete warnings and wanings related to nullable references. There was too many of these warnings. They can be turned on again later to properly deal with them.

MigrationTools.TestExtensions.csproj

This project was completely empty, it was not referenced in solution file, but still was referenced in other projects. It triggered errors in my Visual Studio build.

[MrHinsh]

This looks good.

[MrHinsh]

I appreciate the discrete changes...