Make it easier to use typst output with --citeproc

[jgm]

Currently if you use -t typst with --citeproc, you'll get a typst #bibliography and native typst citations, which isn't desired. See #10655. Using -t typst-citations is required: this disables the native typst citation syntax. But this is pretty counterintuitive and is likely to trip up users. It would be good to disable the citations extension in the typst writer automatically when --citeproc is used.

[jgm]

I note the following in the man page for --citeproc:

Note: if your target format is markdown, org, or typst,
you will need to disable the citations extension (e.g., -t markdown-citations) to see the rendered citations and
bibliography. Otherwise the format's own citation syntax will
be used.

So I think this is already well documented, and maybe that's enough.

[jgm]

I decided to disable the citations extension in the writer when --citeproc is specified; this will fix the problem.