Is your feature request related to a problem? Please describe.

Many of C# libraries use extension methods to extend functionality of the existing types, and the C# language service can list all of them out, no matter whether the namespace is opened. Users doesn't care about which namespace containing the extension method they want to use.

Describe the solution you'd like

Show all extension methods in completion list, no matter whether the namespace is opened.

Describe alternatives you've considered

Manually find out which namespace containing the extension method, and open it.