Improved .editorconfig intellisense

[vsfeedback]

In Visual Studio 16.0.3, there is limited intellisense support for .editorconfig files. One challenge is that as new Visual Studio versions may add support for new .NET or C# options, it's hard to know what existing .editorconfig options are supported but not yet included in the .editorconfig file. The one-time export of VS settings into a .editorconfig files doesn't help with ongoing maintenance.

It would be nice if intellisense within a .editorconfig file only suggested options that are not already included within each respective file type. For example, suppose under "[*.cs]" if we already have "dotnet_style_qualification_for_event = true:warning". If we hit control-space, "dotnet_style_qualification_for_event" should not appear as an option.

An alternate approach could be to automatically add unspecified styles to .editorconfig files as Visual Studio support is available.

This issue has been moved from https://developercommunity.visualstudio.com/content/idea/557790/improved-editorconfig-intellisense.html
VSTS ticketId: 887728
These are the original issue comments:

Jane Wu [MSFT] on 5/6/2019, 02:05 AM (73 days ago):

Thank you for taking the time to provide your suggestion. We will do some preliminary checks to make sure we can proceed further. We'll provide an update once the issue has been triaged by the product team.

[sharwell]

We are hoping to include @madskristensen's EditorConfig extension in future releases of Visual Studio, which addresses this issue and others.

[sharwell]

Related to #19055