Potentially update CSS nesting for additional spec changes

I just became aware that Chrome recently changed how CSS nesting works again: https://web.dev/blog/css-nesting-cssnesteddeclarations. Specifically rules consisting of mixed nested and non-nested CSS now resolve in a different order than before. This should be investigated to understand exactly what changed and what should be done about it.

Start with the linked web.dev article and inspect esbuild's CSS nesting implementation to determine which mixed nested and non-nested rule behavior changed. Compare the current behavior with the updated specification, then establish whether an implementation change or regression tests are needed; done means the required follow-up is identified and covered appropriately.

Written by the indexing model from the issue text.