FR: Add a --watch flag

[jamesrswift]

The typst-test program executes sufficiently rapidly that power users may benefit from a --watch flag that runs the test suite when it detects a change in any project files.

It may be more involved than just adding a flag, however, as people are likely to want to exclude or include specific files or filetypes, so a configuration service may also be needed (which is a lot of bloat)

[tingerrr]

This should come for free when I get #13 done. But work on that has been slow as I am primarily focused on Typst projects that I use while writing my master's thesis.