Allow getting extensions by partial name.

[bobdenotter]

I can foresee a lot of people getting stuck when getting (the config of) extensions, if it requires the full name. For example, before you had to do this:

/** @var ExtensionRegistry $extensionRegistry */
$extensionRegistry->getExtension('Bolt\\BoltForms\\Extension');

Now you can do this:

/** @var ExtensionRegistry $extensionRegistry */
$extensionRegistry->getExtension('Bolt\\BoltForms');

// or even
$extensionRegistry->getExtension('BoltForms');

[I-Valchev]

Neat! 👍