kitty: 0.17.4 -> 0.18.0

https://github.com/kovidgoyal/kitty/releases/tag/v0.18.0

I submitted a patch to kitty, so that the library paths can now be passed as arguments to the build system, eliminating the need for patching. This should reduce the required maintenance effort in the future.

This commit removes the dependency on `which`, which is no longer necessary since 5e4fe584. It was introduced in 481e61fa.

The configure phase is now ignored because kitty has no configure script.

The kitty tests are now run after compilation.

The zsh completions no longer need to be invoked with `source` since 38eb7378, so remove the outdated comment.

There is now a link to the changelog.