Function merged_allow_plugins 

pub fn merged_allow_plugins(
    project: Option<&Value>,
    global: Option<&Value>,
) -> Option<Value>

Config::merge for allow-plugins: the project value replaces the global one unless both are objects, in which case array_merge($project, $global, $project): the project keys first, in their order, then those only the global config brings. Order matters: the first matching rule decides.