Improve experimental theme/module installation warning

Problem/Motivation

\Drupal\system\Form\ModulesListConfirmForm::buildMessageList() contains:

$items[] = $this->formatPlural(count($dependencies), 'You must enable the @required module to install @module.', 'You must enable the @required modules to install @module.', [

and
\Drupal\system\Form\ThemeExperimentalConfirmForm::buildForm() contains:

$items[] = $this->formatPlural(count($dependencies), 'You must enable the @required theme to install @theme.', 'You must enable the @required themes to install @theme.', [

This results in:

You must enable the foo, bar, baz themes to install Claro.

This is awkward/bad English.

Proposed resolution

Improve to:

You must enable the foo, bar and baz themes to install Claro.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet