Support custom code fences for rendering examples

[vecerek]

What is the problem this feature would solve?

Right now, all examples are rendered using the standard ```ts and ``` code fences. This is not bad but sometimes the examples within the API reference could use more interactivity. For example, some examples could benefit from being rendered using shiki twoslash.

What is the feature you are proposing to solve the problem?

This feature would allow people to control the exact fences to be rendered on a per example basis.

What alternatives have you considered?

Global-config to control code fences for all examples but I abandoned the idea because having too many shiki twoslash code snippets could have a serious impact on build times in certain static site generators