Investigate how users can get CSS suggestions

[janechu]

Description

There are a few behavioral issues that could potentially improve the authoring experience with regards to styling. These include CSS variables, if these are defined how can they be taken advantage of, and class names.

Requirements

• Investigate how to add CSS variables to the CSS Control as dropdown suggestions
• Investigate how to add CSS classes as suggestions to HTML in the Monaco Editor and Form
• Investigate how to add CSS defined style to the Form, if it can be identified from an external source such as a stylesheet - see https://github.com/microsoft/fast-creator/issues/126