Dark syntax highlighting theme for Web IDE

Problem to solve

GitLab currently supports a Dark syntax highlighting preference for viewing blobs in the repository. However, this is not supported when editing code inside of the Web IDE.

Proposal

Inside of the Web IDE we should support Dark syntax preference based on #195701 (comment 268717800)

Important Note: In scope for this is only the code editing area. In a future iteration we'll work to expand themes to more of the viewable area in the Web IDE.

Further details

Only the code editing pane will be in scope for this change, all sidebars will be out of scope for the initial implementation so that we can continue to evaluate the change across the application.

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.