Issue #3270345: Support for empty self-closing tags like [hr/]

src/Parser/Tree/TagElement.php

src/Parser/Tree/TagElementInterface.php

src/Parser/XBBCodeParser.php

src/Plugin/Filter/XBBCodeFilter.php

src/PreparedTagElement.php

standard/config/install/xbbcode.tag.hr.yml

tests/src/Kernel/XBBCodeFilterTest.php

...code_test_plugin/src/Plugin/XBBCode/XBBCodeTestPlugin.php