JSON literal expressions

Despite being out of vogue, XML is still in use in tons of businesses and many VB customers I talk to continue to enjoy the feature. It's natural to ask whether we can duplicate the wins here with JSON. I was a little hesitant on JSON until I saw that SQL Server 2016 has built-in JSON support.


I've implemented a working prototype [here](https://github.com/AnthonyDGreen/roslyn/tree/features/vb-json) though it's out of date and needs updating to work with the latest Roslyn sources/VS2017. It's build around JSON.NET. The syntax is just JSON:

``` VB.NET
Dim obj As JObject = { "messageType": "", "content": "SomeText" }
```

And like XML it supports embedded expressions:

Any value not native to JSON (string, number, JSON object, JSON array, true, false, null) is an embedded expression:

``` VB.NET
Dim obj As JObject = { "name": }
```

Any key not a string is an embedded expression.

``` VB.NET
Dim obj As JObject = { : "" }
```

Any member not a key-value-pair is an embedded expression.

``` VB.NET
Dim obj As JObject =
{
"key1": "value1",
from kvp in dictionary select (e.key: e.value)
}
```

The big "win" uses I imagine are copying and pasting a JSON example/message from some website into your code and then substituting in your own dynamic values with embedded expressions. I hope this could be a powerful tool for communicating to REST services since users won't have to create separate POCO objects and serialize them or manually construct JSON.NET JObjects:

The second big win use is pairing JSON literals with pattern matching, enabling the developer to decompose a JObject off the wire without deserializing to a .NET Object.

``` VB.NET
Select Case message
Case Match { "messageType": "ack",
"timestamp": Date(d) }

Console.WriteLine($"Message receipt received at {d}")
End Select
```

No need for any special member access syntax since the ! operator works natively with JSON.NET objects. We should consider some sort of pattern-based support so the feature could target either JSON.NET or Windows.Data.Json but if we can only do one, JSON.NET is preferred.