Draft: part: allow configuring file reload delay

typst is fast enough that incremental renders for a tiny document take around 60ms. That is, the time from starting typst (warm start, but not incremental build) to the last modification timestamp of the PDF file is 60ms. A 750ms delay in okular before the file is considered stable is much higher than the actual runtime of typst and this delay makes the automatic reload slow.

A temporary workaround is running echo test.pdf | entr okular --unique --noraise /_ to automatically trigger a reload in okular as soon as possible.

This commit allows setting the file reload delay.