Update typst.qmd

[mpr1255]

From this discussion we learned of the undocumented QUARTO_TYPST env var. This got me out of a jam recently and I was able to finally compile my thesis using some of the par options in 0.12. Nothing else broke.

So this PR is just to add an explainer for advanced users as to how they can point quarto to a different typst compiler if they so choose.

[cscheid]

This feature is intentionally not documented. It exists for third-party packagers, not for users. So I don't want to spend precious documentation inches on discussing it. Instead, I'd prefer to document how document rendering can stop at .typ instead of .pdf:

format:
  typst:
    output-ext: typ