Core: New hook before language class is set

[RunDevelopment]

This adds a new hook as described in #2665.

I named the new hook before-set-language instead of highlight-start for 2 reasons:

1. There are hooks in highlightAllUnder that run before this hook.
2. We may add hooks in highlightElement before this one in the future.

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I also extracted the code that sets the language-xxxx class of an element into its own function.

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This resolves #2665.

[github-actions]

JS File Size Changes (gzipped)

A total of 1 files have changed, with a combined diff of +12 B (+0.4%).

Details

file	master	pull	size diff	% diff
components/prism-core.min.js	3.03 KB	3.04 KB	+12 B	+0.4%

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