refactor: move the smtp option parsing into a pure class

[YvesCesar]

Stacked on #5.

The two decisions the plugin makes on its options, whether smtp_auth means "on" and whether an SSL option turns a certificate check on, off or leaves the default, were written inline in the phpmailer_init callback, so they could only be tested through WordPress.

• Adds src/OptionValue.php, two pure static methods: smtp_auth() keeps the existing rules (TRUE, ON, 1, OK, any non-zero number), and ssl() returns true, false or null for "keep the PHP default". The main file loads the class with require_once; the Composer autoloader only serves PHPUnit and PHPStan.
• librecode_simple_smtp_mail_sender() now only reads the options and hands them to OptionValue.
• Adds a unit suite (phpunit.xml.dist, composer test:unit) that runs without WordPress or a database, with named data providers for both methods. composer test runs unit, then integration.

No behaviour change: the integration suite from #5 passes unchanged. composer ci passes with 31 unit and 38 integration tests.