Core: Added setLanguage util function

[RunDevelopment]

This PR adds a setLanguage to Prism.util.

I also changed the regex we use to detect language-xxxx classes. It now uses whitespace boundaries instead of word boundaries. This is consistent with the className/classList spec.

(I also slightly simplified the getLanguage language.)

The File Highlight plugin now uses Prism.util.setLanguage instead of its own implementation.

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This fixes #2383.

[github-actions]

JS File Size Changes (gzipped)

A total of 2 files have changed, with a combined diff of -59 B (-1.4%).

file	master	pull	size diff	% diff
components/prism-core.min.js	3.08 KB	3.09 KB	+7 B	+0.2%
plugins/file-highlight/prism-file-highlight.min.js	1.14 KB	1.08 KB	-66 B	-5.8%

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