Clean up Previewers' page (#1630)
Clean up Previewers' page (#1630)

This PR makes some changes to the [Previewers plugin's page](https://prismjs.com/plugins/previewers/).

1. Format the code examples to look nicer.
2. Fix the example of the 'Disabling a previewer' section.
3. Highlight API snippets as JavaScript.
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