move typst-gather out of core build

[gordonwoodhull]

As @cderv pointed out, we should really have typst-gather in its own repo:

• It's slow to build.
• We don't really need to interleave its development with Quarto's.
• It's actually simpler to use Rust GitHub tools rather than building it ourselves.
• We can bucket it like everything else.

I won't get to it this week; filing an issue for later in the release.