Add a helper function: now()

Problem/Motivation

Whenever I need to save a date+time programmatically, I write

$now = (new \Drupal\Core\Datetime\DrupalDateTime('now', new \DateTimeZone('UTC')))->format('Y-m-d\TH:i:s');
# or
$now = \Drupal::service('date.formatter')->format(time(), 'custom', 'Y-m-d\TH:i:s', 'UTC');

Even with the help of VS Code and/or a use statement, it's quite a lot to remember and rather complicated.

Imagine if I could just ask for drupal_get_now() and get all the goodness of (new \Drupal\Core\Datetime\DrupalDateTime('now', new \DateTimeZone('UTC')))->format('Y-m-d\TH:i:s') in one fell swoop!

Setting the date field on a node would be as easy as
$order_node->set('field_paid_date', drupal_get_now());

I bet

drupal_get_now('date'); // just the date
drupal_get_now('timestamp'); // timestamp

would be useful to lots of people as well.

Steps to reproduce

Try saving a date programmatically.

My guess is that 90% of the dates my sites save programmatically are 'now'.

Proposed resolution

Place the function in the datetime.module.

Remaining tasks

Merge request 10200 needs review and tests.