feat: replace Sass with modern CSS
Almost all the Sass features I was using can now be written in native
CSS. Some required restructuring the HTML a little bit to make the most
of the cascade but I think it's resulted in a more semantically correct
document. Especially happy with how I have replaced explicit 'left' and
'right' project IDs with a declarative approach using nth-child.
Getting rid of Sass (and autoprefixer/postcss which isn't really needed
these days) has massively improved build times as we no longer need to
shell out to JS.
I needed to bump esbuild to a newer version in order to support CSS
nesting, so also had to update its config file to accomodate API
changes.