Feature allow configuration extension from a different base file

[GuillaumeOnepilot]

I have a use case where my company uses a global pint config, this work fine if the config doesn't need to change per project, but if we need to have project specific configuration, it becomes impossible to have them isolated.

It partially solves #174

I purposefully chose to only allow 1 extend to avoid recursion issues (though it could be fixed through a fixed set of extent)

[nunomaduro]

Maybe this should be array_merge_recursive? What do you think?

Also, can you add more tests that expand on this—to determine exactly which configuration will be used, especially when dealing with nested JSON files like this:

{
    "preset": "laravel",
    "rules": {
        "array_push": true,
        "backtick_to_shell_exec": true,
        "date_time_immutable": true,
        "declare_strict_types": true,
        "lowercase_keywords": true,
        "lowercase_static_reference": true,
        "final_class": true,
        "final_internal_class": true,
        "final_public_method_for_abstract_class": true,
        "fully_qualified_strict_types": true,
        "global_namespace_import": {
            "import_classes": true,
            "import_constants": true,
            "import_functions": true
        },
        ...