Change keyup to keydown to avoid MacOS conflicts with cmd-c

[Brixy]

Hi and thank you for this script—and the excellent Min browser!

Using cmd-c on MacOS sometimes works as expected (copy marked text) and sometimes it acts like a regular c press.

Replacing

document.addEventListener('keyup', function (e) {

with

document.addEventListener('keydown', function (e) {

solved the problem.

I did not make a pull request because I could only test this on MacOS.