feat: 📄 integrate llms.txt generation functionality

[okineadev]

Description

Integrated llms.txt generation functionality from the vitepress-plugin-llms

Now you can enable llms.txt generation using the llms parameter:

llms: true

You can also pass parameters there:

llms: { workDir: 'en' }

This is a rough draft for now, I just moved the code from vitepress-plugin-llms

I also thought about just importing the plugin and adding it to the list of built-in plugins, but I thought about it and decided that it wouldn't make much sense, because if this feature is integrated directly into VitePress, then VitePress should maintain it accordingly

Ready for further suggestions and changes.

Linked Issues

Closes #4590

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Tip

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Don't merge this PR yet

[okineadev]

I am temporarily closing it due to the fact that this is not planned to be done in the near future.