Enable Comrak support for style alerts in Markdown

[kbdharun]

This PR enables alerts (for GitHub style alerts kivikakk/comrak#519) and multiline_block_quotes (for GitLab multiline blockquotes kivikakk/comrak#521) Comrak extensions that were added/modified in the recent release https://github.com/kivikakk/comrak/releases/tag/v0.34.0, to the Crates Markdown renderer.

Related discussion: #8506

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While I was able to build the frontend as well as backend via Docker, I have not tested the rendering with a sample crates package (due to my unfamiliarity with the codebase). Any help with testing this and adding test cases for style alerts would be appreciated.

Extension docs:

https://docs.rs/comrak/0.34.0/comrak/struct.ExtensionOptionsBuilder.html#method.alerts
https://docs.rs/comrak/0.34.0/comrak/struct.ExtensionOptionsBuilder.html#method.multiline_block_quotes

[Turbo87]

https://github.com/rust-lang/crates.io/blob/main/tests/acceptance/readme-rendering-test.js
http://localhost:4040/tests?filter=Acceptance%20|%20README%20rendering

Any help with testing this and adding test cases for style alerts would be appreciated.

we need two test cases:

1. checking the conversion of the markdown alert into an HTML alert. this should be added at the bottom of the file in the crates_io_markdown crate, which already has a bunch of similar tests.

2. checking the styling of the HTML alerts. you can find a basic smoke test for the rendering at https://github.com/rust-lang/crates.io/blob/main/tests/acceptance/readme-rendering-test.js. if you run pnpm start you should be able to visit http://localhost:4040/tests?filter=Acceptance%20|%20README%20rendering and see it in action. if you replace wait percySnapshot(assert); with await this.pauseTest(); you can pause the test and manually check if the styling matches your expectation.