Svelte 5: Nested CSS does not work with the `:has` pseudo-selector.

[Lofty-Brambles]

Describe the bug

Svelte 5 (and most probably 4) seems to be unable to parse the & symbol within a :has pseudo-selector.
It throws a warning: Unused CSS selector "header:has(> &)".
It can be rewritten easily in the minimal reproduction provided; however, it is something that Svelte is unable to parse right now.

Reproduction

This is an example of a component with the aforementioned issue

Logs

No response

System Info

System:
    OS: Linux 5.10 Ubuntu 22.04.4 LTS 22.04.4 LTS (Jammy Jellyfish)
    CPU: (4) x64 Intel(R) Core(TM) i3-2310M CPU @ 2.10GHz
    Memory: 3.20 GB / 6.14 GB
    Container: Yes
    Shell: 5.8.1 - /usr/bin/zsh
  Binaries:
    Node: 18.15.0 - ~/.nvm/versions/node/v18.15.0/bin/node
    npm: 9.5.0 - ~/.nvm/versions/node/v18.15.0/bin/npm
    pnpm: 8.5.1 - ~/.local/share/pnpm/pnpm
  npmPackages:
    svelte: 5.0.0-next.244 => 5.0.0-next.244

Severity

annoyance