There is no auto-complete for events ( event += )

## Environment data
`dotnet --info` output:
Version: 5.0.200

VS Code version:
1.54.3 (system setup)
C# Extension version:
v1.23.9

## Steps to reproduce

create a new C# class and assign it some events, for example:

```csharp
public class Test
{
public delegate void _foo();
public event _foo Foo;
}
```
and then create another class with a main method, create the Test class object and try to subscrive to the event:

```csharp
public class MainClass
{
public Test mytest = new Test();
public void MainMethod()
{
mytest.Foo += // here is where you sould get the AutoCompletion suggestion with the "hint" icon or automatic auto complete when pressing double tab
}
}
```

## Expected behavior

there is actual auto completion suggestion

## Actual behavior

No auto completion at all and you have to create subscription for the event manually.