refactor remark-code-region.ts to suport nested code regions

[zachwaffle4]

Description

This allows us to have nested code regions, eg

// [translation2d]
public class Translation2d {
  // [translation2dFields]
  final double x;
  final double y;
  // [/translation2dFields]

// blah blah the rest of the class
}
// [/translation2d]

Meta

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• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why, including events that led to this PR
• Pull Request has been linted and formatted and follows the styleguide

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