Plugins: Consistent Prism check

[RunDevelopment]

Here is a fix for something that bugged me for a long time. All of Prism's plugins now use the same check to determine whether Prism is loaded.

The new check is a simple typeof Prism === 'undefined'. The self variable is now completely ignored.

Checks for DOM access are similar: typeof document === 'undefined'.

I also made the formatting of the check consistent with one empty line before and after it.

[github-actions]

JS File Size Changes (gzipped)

A total of 25 files have changed, with a combined diff of -157 B (-0.7%).

file	master	pull	size diff	% diff
plugins/autolinker/prism-autolinker.min.js	617 B	595 B	-22 B	-3.6%
plugins/autoloader/prism-autoloader.min.js	2.21 KB	2.2 KB	-9 B	-0.4%
plugins/command-line/prism-command-line.min.js	948 B	944 B	-4 B	-0.4%
plugins/copy-to-clipboard/prism-copy-to-clipboard.min.js	715 B	711 B	-4 B	-0.6%
plugins/custom-class/prism-custom-class.min.js	296 B	276 B	-20 B	-6.8%
plugins/data-uri-highlight/prism-data-uri-highlight.min.js	611 B	590 B	-21 B	-3.4%
plugins/download-button/prism-download-button.min.js	272 B	267 B	-5 B	-1.8%
plugins/file-highlight/prism-file-highlight.min.js	944 B	943 B	-1 B	-0.1%
plugins/filter-highlight-all/prism-filter-highlight-all.min.js	439 B	440 B	+1 B	+0.2%
plugins/highlight-keywords/prism-highlight-keywords.min.js	145 B	123 B	-22 B	-15.2%
plugins/inline-color/prism-inline-color.min.js	601 B	595 B	-6 B	-1.0%
plugins/jsonp-highlight/prism-jsonp-highlight.min.js	1.26 KB	1.26 KB	-4 B	-0.3%
plugins/keep-markup/prism-keep-markup.min.js	601 B	598 B	-3 B	-0.5%
plugins/line-highlight/prism-line-highlight.min.js	1.43 KB	1.43 KB	-5 B	-0.3%
plugins/line-numbers/prism-line-numbers.min.js	1.12 KB	1.11 KB	-4 B	-0.4%
plugins/match-braces/prism-match-braces.min.js	853 B	848 B	-5 B	-0.6%
plugins/normalize-whitespace/prism-normalize-whitespace.min.js	1.08 KB	1.1 KB	+19 B	+1.8%
plugins/previewers/prism-previewers.min.js	2.98 KB	2.98 KB	-3 B	-0.1%
plugins/remove-initial-line-feed/prism-remove-initial-line-feed.min.js	239 B	238 B	-1 B	-0.4%
plugins/show-invisibles/prism-show-invisibles.min.js	364 B	344 B	-20 B	-5.5%
plugins/show-language/prism-show-language.min.js	2.32 KB	2.31 KB	-4 B	-0.2%
plugins/toolbar/prism-toolbar.min.js	751 B	746 B	-5 B	-0.7%
plugins/treeview/prism-treeview.min.js	416 B	432 B	+16 B	+3.8%
plugins/unescaped-markup/prism-unescaped-markup.min.js	514 B	511 B	-3 B	-0.6%
plugins/wpd/prism-wpd.min.js	1.3 KB	1.28 KB	-22 B	-1.7%

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