Add support for adding rich annotations to the workflow

[glopesdev]

This PR implements a proposal for adding rich annotations to the workflow. It was designed to meet the following goals:

• Require no changes to the current language syntax and semantics
• Allow annotation of both workflow sections and individual operators
• Support rich annotations in Markdown / HTML

The current design includes two major additions to the core and IDE.

AnnotationBuilder

This is a new operator which will behave in the same way as externalized properties. By itself it generates no observable output and no expression, but can be attached to other operators to represent an association of the annotation to that operator. The Text property represents the contents of the annotation itself, which are encoded as a CDATA section in the workflow XML format. This section can include plain text or rich Markdown (CommonMark) / HTML to be formatted and rendered by the IDE, or simply read as part of the text document.

WebView Panel

To render the annotations, we use Markdig to preprocess the annotation text into HTML, and WebView2 as the rendering framework. A new browser panel is added to each workflow editor canvas that is revealed when annotations are launched. This will render the preprocessed annotations but also function as a full in-app browser for documentation and future purposes.

Fixes #750
Closes #1170