[Proposal] Using dictionaries as extension properties

I suggest to use dictionaries as a simple value properties like this:

```VB.NET

Dim Values As new Dictionary(of String, Integer)

Sub Main()
Values("Adam") = 1 ' as usual
"Adam".Value = 1 ' extension to a literal
Dim Adam = "Adam"
Adam.Value = 1 ' extension to a variable
End Sub
```

Of course this can also used to add extension methods:
```VB.NET

Dim IncrementFuncs As new Dictionary(of String, Func(Integer))

Sub Main()
Dim Adam = "Adam"
Adam.Increment = Function(n) n + 1
Console.WriteLine(Adam.Increment(1))
End Sub
```

Issue 570 contains proposal examples but names no files, entry points, or tests. Start by reading the requested dictionary-backed extension property syntax and determine the intended language behavior; done would require an accepted design with defined implementation and test scope.

Written by the indexing model from the issue text.