[8.x] Add method to MigrationsStarted/MigrationEnded events

[kohenkatz]

The MigrationStarted and MigrationEnded events that are fired for individual migrations have a method property that can have the values up or down to indicate whether this is a migration or a rollback.

However, the MigrationsStarted and MigrationsEnded (plural) which run at the beginning and end of a set of migrations do not have this property.

This PR adds the method property to those events as well.

The intended use-case of this change is to allow pre- and post-migration code to run better. For example, consider this code:

// Check if the proper PostgreSQL extensions are loaded.
// Throws an exception if an extension is missing or the PostgreSQL version is too old.
Event::listen(function (MigrationsStarted $event) {
    Artisan::call('project:ensure-extensions', ['--ansi' => true]);
    echo Artisan::output();
});

// Ensure initial required data is present in the database after migrations are complete.
// See https://twitter.com/taylorotwell/status/1387766514674192384 (thread)
// for the inspiration behind this code.
Event::listen(function (MigrationsEnded $event) {
    Artisan::call('project:ensure-db', ['--ansi' => true]);
    echo Artisan::output();
});

There are two problems with the above code when running migrate:rollback:

1. (minor) It is unnecessary to run project:ensure-extensions because the database tables already exist so the requirements were clearly met.
2. (major) project:ensure-db will crash because it is trying to write to database tables that no longer exist.

With this PR, the code above can be modified like this:

// Check if the proper PostgreSQL extensions are loaded.
// Throws an exception if an extension is missing or the PostgreSQL version is too old.
Event::listen(function (MigrationsStarted $event) {
    if ($event->method === 'up') {
        Artisan::call('project:ensure-extensions', ['--ansi' => true]);
        echo Artisan::output();
    }
});

// Ensure initial required data is present in the database after migrations are complete.
// See https://twitter.com/taylorotwell/status/1387766514674192384 (thread)
// for the inspiration behind this code.
Event::listen(function (MigrationsEnded $event) {
    if ($event->method === 'up') {
        Artisan::call('project:ensure-db', ['--ansi' => true]);
        echo Artisan::output();
    }
});