"None" syntax highlighting theme for Web IDE

Problem to solve

GitLab currently supports a None syntax highlighting preference (lack of a theme) for viewing blobs in the repository. However, this is not supported when editing code inside of the Web IDE.

Proposal

Add a theme in the Web IDE for Monaco that doesn't use any styling customization.

Documentation

Documentation should be updated for the Syntax highlighting theme to indicate which Themes are also supported in the Web IDE and that it also applies to the Web IDE.