Previewers: Use classList instead of className

[RunDevelopment]

I changed Previewers to use classList instead of className. This makes the code easier to understand and less error-prone. Since classList.add will automatically filter dup classes, #2699 is also fixed.

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This fixes #2699.

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of -40 B (-1.3%).

file	master	pull	size diff	% diff
plugins/previewers/prism-previewers.min.js	3.02 KB	2.98 KB	-40 B	-1.3%

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