syntax highlighter: support dark theme switching

[codymullins]

The current implementation of the syntax highlighter does not support switching to a light theme when the overall theme of the library changes.

• Add Parameter AutomaticDarkMode
• AutomaticDarkMode should default to false and be opt-in since this introduces more JS and could have a minor impact on performance.
• If AutomaticDarkMode is true, the component should automatically switch between light/dark themes.
• Add parameter to manually specify dark vs light mode.
• This parameter should default to dark mode.

Outstanding questions

• Prism, the library behind the syntax highlighter component, utilizes the presence of the CSS on the index.html page. Need to investigate the best way to integrate this appropriately.
• There could be an opportunity to better support theming. Themes in general utilize this CSS on the index page.