feat(plugin-prismjs): add `prismjs` classname to `<pre>`

[pengzhanbo]

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

• [x] Read the Contributing Guidelines.
• [x] 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
• [x] New feature
• [ ] Other

Description

- <pre class="language-typescript">
+ <pre class="prismjs language-typescript">

[coveralls]

Pull Request Test Coverage Report for Build 9280344640

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 56.484%

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Totals
Change from base Build 9280295370:	0.02%
Covered Lines:	995
Relevant Lines:	1575

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💛 - Coveralls