Markdown: Workaround for double wrap hook

[RunDevelopment]

This is a workaround for #2716.

The problem in #2716 is that Markdown's wrap hook function might be added multiple times because the language might be reloaded. I did not account for that when writing the function, so this is a fix for that. However, it does not solve the underlying issue.

This workaround should be kept even after #2716 was fixed because other plugins might modify env.content before Markdown and add custom HTML.

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of +7 B (+0.4%).

file	master	pull	size diff	% diff
components/prism-markdown.min.js	1.76 KB	1.76 KB	+7 B	+0.4%

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