feat: support astro-xmdx as an alternative MDX integration

[jp-knj]

• Recognize astro-xmdx as a valid MDX integration, skipping the automatic addition of @astrojs/mdx when it is already present
• Prevents duplicate MDX integrations from conflicting

Test plan

• Manually verified that astro-xmdx users no longer get @astrojs/mdx auto-added

Related

• Integrate astro-xmdx into withastro/docs docs#13274

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