Add shiki lang path compat

[bluwy]

Changes

fix #8970

Shiki supports languages with the path prop, which loads language JSON files, relative to the package root (in our case, the Astro package 😬). It's a bit of an ugly compat, but they should be removed in Astro 4.

Testing

added new test based on the issue repro.

Docs

n/a

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[ematipico]

It looks good to me. Is there any chance that we could add this feature to shikiji directly? Are the maintainers willing to accept such change?

[bluwy]

I don't think Anthony (shikiji author) will be adding this. Its removal is actually the goal of shikiji to rely on ESM imports instead so the code is portable. The compat code we add here will bring the caveats of shiki before where it doesn't work too well in SSR.

Ideally they should move away from using path. I've commented some additional information at #8970 (comment)

[ematipico]

That makes sense, thank you for clarifying!