Add using keyword completion provider

[allisonchou]

See Gen's comment: dotnet/vscode-csharp#6441 (comment)

This should allow us to get rid of the VS Code-specific hack in dotnet/vscode-csharp#6441 and instead make a more generalized hack that works for both VS and VS Code.

[davidwengier]

👍

We should be able to just add a new element to this dictionary: https://github.com/dotnet/razor/blob/main/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Completion/DirectiveCompletionItemProvider.cs#L48

[alexgav]

We added appropriate entries in directive completion code, removed VSCode-only hack added last year.