Bug: Add remark-slug and remark-external-links to Vite builder

[JReinhold]

Closes #20519

What I did

This PR adds remark-slug and remark-external-links plugins to the MDX plugin in the Vite builder. The plugins are already added to addon-docs, however that only has an effect in Webpack projects. The changes to the mdx plugin here are direct copy-paste from https://github.com/storybookjs/storybook/blob/next/code/addons/docs/src/preset.ts#L55-L66

Not having these plugins caused the issue where headings wouldn't automatically get ids assigned, and thus no anchors. This fixes that.

In the near future we probably want to move all MDX handling from the Vite builder to the docs addon, to not force Vite users to have all those dependencies.

How to test

This is not an issue in autodocs, only standalone MDX files.

Create a standalone MDX file with the following content:

# This is H1

{"text".repeat(1000)}

## This is H2

More text

Hover over the headings and see the anchor icon appear. Click the H2 heading, copy the URL and open it in a new tab. See that it scrolls down to the H2 when opened.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[socket-security]

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Pull request alert summary

Issue	Status
Install scripts	✅ 0 issues
Native code	✅ 0 issues
Bin script confusion	✅ 0 issues
Bin script shell injection	✅ 0 issues
Shell access	✅ 0 issues
Uses eval	✅ 0 issues
Unresolved require	✅ 0 issues
Invalid package.json	✅ 0 issues
HTTP dependency	✅ 0 issues
Git dependency	✅ 0 issues
GitHub dependency	✅ 0 issues
New author	✅ 0 issues
Potential typo squat	✅ 0 issues
Known Malware	✅ 0 issues
Telemetry	✅ 0 issues
Protestware/Troll package	✅ 0 issues

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📊 Modified Dependency Overview:

➕ Added Package	Capability Access	+/- Transitive Count	Publisher
remark-slug@7.0.1	None	+0	wooorm
remark-external-links@9.0.1	None	+1	wooorm

[JReinhold]

1. Sort alphabetically
2. Add space-separated-tokens and is-absolute-url, ESM only modules used by remark-external-links