Update shikiPlugin.ts

[jianjianai]

添加一个 extendsHighlighterConfigOptions 钩子。( shiki 在加载自定义主题时需要用到 highlighter 对象)

[Mister-Hope]

docs need update, and an example code with this option is needed in pr body.

[coveralls]

Pull Request Test Coverage Report for Build 9089456277

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 49.759%

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Totals
Change from base Build 9075650266:	0.0%
Covered Lines:	681
Relevant Lines:	1238

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💛 - Coveralls

[pengzhanbo]

近期 @vuepress/plugin-shiki 会有一次比较大的更新，且看起来你的这个 PR 提出的功能已在 #137 中实现， 有一个新的配置选项 shikiSetup: (shiki: Highlighter) => void | Promise<void> ，它应该能够满足你的需求。