feat(core)!: make Environment its own source of truth

[innocenzi]

Because injecting Environment is more discoverable and a better DX than injecting AppConfig.

public function __construct(
    private readonly Environment $environment,
) {}

EDIT: so actually, as specified in my comment below, I completely dissociated AppConfig from Environment:

• The booting process uses Environment::fromEnv indirectly through AppConfig's constructor when resolved from the container
• Specifying the environment property of the AppConfig in userland will not behave as expected due to that
• It makes sense that Environment is its own source of truth in the container, it's even easier to spoof in tests

[innocenzi]

Actually, I think there is an opportunity to refactor the environment outside of AppConfig. The kernel booting process uses the ENVIRONMENT variable itself indirectly anyway, because AppConfig gets resolved every time with null values, and its environment property gets initialized with ::fromEnv, so even if users set a custom environment value in their config, that wouldn't work.