fix: add md/mdx into KNOWN_ASSET_TYPES for @mdx-js/rollup

[JounQin]

Description

close #15281

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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I didn't include declare module for *.md and *.mdx because it is bundled to use @types/mdx in @mdx-js/rollup.

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[sapphi-red]

This will make import foo from './foo.md'; console.log(foo) to return a URL. I guess that's not what you want.

[JounQin]

This will make import foo from './foo.md'; console.log(foo) to return a URL. I guess that's not what you want.

Oh... Right! This is exactly #15284!

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So how can we fix #15281 do you think?

[JounQin]

close in favor of #15286

[sapphi-red]

I guess #15286 will fix #15281. I'm going to take a look.

[JounQin]

I guess #15286 will fix #15281. I'm going to take a look.

Yes, I confirm that patch works to me.