Added basic syntax highlighting functionality for DS code in CBN

This submission adds the Syntax highlighting definition (.xshd) file for DesignScript syntax and integrates it with the new AvalonEdit text editor. It enables basic syntax highlighting of DS keywords, primitive types, and comments.

**TODO:** Imported types need to be cycled through and added to the keyword list of the syntax definition to appropriately highlight imported types. Doing this can give a basic indication to users by distinguishing between valid and invalid types used in their code as only the imported types will be highlighted. This will come in a subsequent submission.

**Note:** It does not include semantic analysis for syntax error or contextual highlighting. This may be a future effort that would involve integration with the language parser.

This PR can be merged to this branch after review so that any subsequent PR's to this branch can be applied as a  "diff".
Few things below:

1. A single string looks better than two:

    ```cs
    var name = "Dynamo.UI.Resources.DesignScript.Resources.SyntaxHighlighting.xshd";
    var stream = GetType().Assembly.GetManifestResourceStream(name);
    ```

2. ```string highlighting``` should be placed in ```Configurations.cs```.

3. ```stream``` should sound better than ```elem``` as a stream name :)