Show all extension methods in completion list.

**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.