Expose base config to Astro global namespace

[lwhiteley]

Body

I saw the issue but it was closed without further discussion
withastro/astro#3626

Summary

currently astro exposes only Astro.site
but can it be possible to also expose base at that same level so one could use this to create custom link components for mdx instead of copying the base path in every markdown file or astro file

Goals

A concise, bulleted-list outlining the intended goals of this RFC.

• Create a component that handles reading the base path and append to links in one place

Example

Astro.base

[bluwy]

There's an existing discussion at #845

[lwhiteley]

Thanks I missed it