Separated code completion queries into CodeCompletionServices

This pull request separates code completion queries into a `CodeCompletionServices` class and out of the `CodeBlockEditor` UI component so that it can be tested independent of the UI, `DynamoModel` etc. with its own core and own set of libraries. This is part one of a series of submissions involving this change. The PR's that will follow will include calls into `CodeCompletionServices` from the UI as well as rewritten unit tests making use of only `CodeCompletionServices`.

@Benglin please review.
I tend to think the following is more succinct:

```cs
string propertyName = property.PropertyName;
return new CompletionData(propertyName, stub: "", CompletionType.Property);
```

That's my personal preference, feel free to ignore.