Remove cssnano

Previously we were using cssnano to minify our CSS, however, we were
also getting warnings when running `bin/vite build` from esbuild which
was also trying to minify our CSS and unable to parse the output of
cssnano.

Instead of trying to minify our CSS twice, we've decided just to use the
Vite default, which is esbuild. This commit removes cssnano from this
project.

The size of the output CSS is slightly increased as a result of this
change;

- using cssnano the output CSS is 139855 bytes,
- with esbuild the output CSS is 140935 bytes;

but we think it's not a large enough increase to be an issue.

Author: lfdebrux