[7.x] Add methods to determine if service provider is loaded

[antonkomarev]

This PR adds 2 methods to the Application which determines if service provider loaded or not.

if ($app->isProviderLoaded(OptionalServiceProvider::class)) {
    // Do something
}

if ($app->isProviderNotLoaded(RequiredServiceProvider::class)) {
    // Throw exception
}

It might be useful in package development when you want to be 100% sure that provider is loaded. Or when you want to load some extra stuff if some optional providers are loaded.

[antonkomarev]

Only one method was accepted and it was renamed, here is how it will look like:

if ($app->providerIsLoaded(OptionalServiceProvider::class)) {
    // Do something
}

if (! $app->providerIsLoaded(RequiredServiceProvider::class)) {
    // Throw exception
}