This should fix #907.

All tests pass, I thought about adding more tests for various plugins but it seemed overkill, happy to add them if the reviewer thinks otherwise.

There might be a regression here (I don't think so) but just so everyone is aware this will start failing:

$smarty = new Smarty();
$smarty->registerPlugin(Smarty::PLUGIN_FUNCTION, 'customtag', [Functions::class, 'customTag']);
$smarty->display('string:{customTag}');

I have kept the "built-in" tags case-insensitive because they have been relying on them being lowercase, so I think it is sensible to keep it this way.