feat: improve types

[pengzhanbo]

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• Bug fix
• New feature
• Other

Description

improve types.

• @vuepress/plugin-prismjs
• @vuepress/plugin-shiki

[coveralls]

Pull Request Test Coverage Report for Build 9160882066

Details

• 1 of 5 (20.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 56.871%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
plugins/plugin-prismjs/src/node/prismjsPlugin.ts	0	4	0.0%

Totals
Change from base Build 9156227249:	-0.2%
Covered Lines:	993
Relevant Lines:	1567

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💛 - Coveralls