We were already polyfilling `light-dark` for older browsers. However,
that polyfill makes use of native CSS nesting, which is also unsupported
by older browsers. This commit adds the `postcssNesting` transform to
turn that nesting back into flat CSS.
This became a noticable problem since light-dark was added to a critical
core stylesheet in 1ec69c3f2f, which then
caused the whole file to be a syntax error under Safari 16.