refactor(plugin-shiki): code and test refactor

[Mister-Hope]

This avoids errors in unit test:

stderr | plugins/markdown/plugin-shiki/tests/shiki-preWrapper.spec.ts > @vuepress/plugin-shiki > fence preWrapper > :line-numbers / :no-line-numbers > should work properly if `lineNumbers` is enabled by default
[Shiki] 10 instances have been created. Shiki is supposed to be used as a singleton, consider refactoring your code to cache your highlighter instance; Or call `highlighter.dispose()` to release unused instances.

stderr | plugins/markdown/plugin-shiki/tests/shiki-preWrapper.spec.ts > @vuepress/plugin-shiki > fence preWrapper > render whitespace > should work whitespace with `trailing` option
[Shiki] 20 instances have been created. Shiki is supposed to be used as a singleton, consider refactoring your code to cache your highlighter instance; Or call `highlighter.dispose()` to release unused instances.

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