Code completions do not get triggered while typing in the curly brackets of a format string

[natanfudge]

Issue Description

If you type a format string with $"", completions do not get triggered when inside the curly brackets : $"{<shouldTriggerCompleteHereButDoesn't>}". ALSO the curly brackets do not get automatically closed when in the format string:
$"{<shouldCloseBracketButDoesn't>"

Steps to Reproduce

• Create a new C# file and paste this code:

using System.Collections.Generic;

public class test { void foo() { string pleaseautocompletethis = "omnibug"; string y = $"{}"; }

}

- Place the cursor in the curly brackets in `y` and start typing `pleaseauto...`
## Expected Behavior ##
`pleaseautocompletethis` and other valid completions should be suggested. 
## Actual Behavior ##
Code completion is not triggered and nothing is suggested. 

## Second, related bug
If you delete both curly brackets in `y` and type a new one, the bracket will not get closed automatically. 
## Logs ##

### OmniSharp log ###
<details>No output.</details>

### C# log ###
<details>No output.</details>

## Environment information ##

**VSCode version**: 1.32.3
**C# Extension**: 1.18.0

<details><summary>Dotnet Information</summary>
.NET Core SDK (reflecting any global.json):
 Version:   2.1.502
 Commit:    c74ce8f29f

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.1.502\

Host (useful for support):
  Version: 2.1.6
  Commit:  3f4f8eebd8

.NET Core SDKs installed:
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.500 [C:\Program Files\dotnet\sdk]
  2.1.502 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download
</details>
<details><summary>Visual Studio Code Extensions</summary>

|Extension|Author|Version|
|---|---|---|
|auto-using|Fudge|0.7.4|
|cs2ts-vscode-extension|chiadem|0.2.1|
|csharp|ms-vscode|1.18.0|
|csharpextensions|jchannon|1.3.0|
|docomment|k--kato|0.1.6|
|vscode-nuget-package-manager|jmrog|1.1.6|
|vscode-path-tools|cg-cnu|0.0.4|
|vscode-test-explorer|hbenl|2.9.1|
|vscode-todo-highlight|wayou|1.0.4|;

</details>

[alvarohc777]

Hi natanfudge, were you able to solve the issue? I'm having the exact same problem.

[heartacker]

me 2

[JoeRobich]

With the current release, when pressing ctrl+space I see the appropriate completions suggested.