Improve language service display for C#

[brettfo]

VS Code expects markdown for things like hover, signature help, etc., but in C# we generate very poor markdown.

E.g., consider this C# code:

/// <summary>Add two numbers together</summary>
int Add(int a, int b)
{
    return a + b;
}

The markdown we generate is this:

int Add(int a, int b)

> Add two numbers together

Which means the display looks terrible:

We should instead generate something like this (raw):

``` csharp
int Add(int a, b)
```

Add two numbers together

This will give us something much nicer.