Support top-level statements in a single entry-point file

Today the entry point of an app has to be a Sub/Function Main (which has to be in a Class or Module). It would be cleaner for simple apps (and for learning) to just let users write top-level statements in a single file so their first program could be:

```
Console.WriteLine("Hello, World!")
```

This wouldn't be a separate scripting dialect of Visual Basic, but just a special case in the language proper. In order to avoid spaghetti nonsense I think it should be limited to a single file rather than stitching together the statements in a bunch of files in some predefined order (as F# does).

If no Main method is found and a single compilation unit has any top-level statements, those statements shall be considered the body of an implicit Main method. The implicit Main should be async by default (a win for samples) and support exit codes.