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.
If I may suggest this:

```cs
/// Code completion data that typically gets displayed on a list as part of 
/// the auto-completion feature. This class represents a common currency that
/// is view-independent.
```