Restore nested option extensibility for
`programs.firefox.profiles.<name>.extensions` by keeping it as a plain
submodule.
This removes the deprecated `programs.firefox.profiles.<name>.extensions
= [ ... ]` shorthand. Add-ons must now be declared with
`programs.firefox.profiles.<name>.extensions.packages`, while
declarative settings remain under `extensions.settings`.
Add a regression test for downstream `extensions.*` option extension and
a news entry documenting the migration.