Assets are only copied before running `watch`, not during ongoing use of `watch`

[joshuahhh]

Because copying assets to build/assets is done with a prewatch script, it only takes effect right when npm watch is run. If a user adds more assets, they have to restart the npm watch process. (Ideally, a user could run npm watch and then work on their document without having to mess around on the command line anymore.)

[joshuahhh]

For future reference: I've been working around this issue with this command (in fish shell):

while true; ls -d assets/* | entr -d npm run assets; end