Enable self-closing tags

Allow tags to be self-closing, for implementing tags that need tag args but not any inner content. Include a mechanism for tags to configure themselves to allow/forbid/require self-closing tags

No files or tests are named, so first locate the tag parsing and tag-configuration entry points in the repository. Define the expected behavior for tags with arguments and no inner content, including configurable allow, forbid, and require modes, then add coverage for each mode.

Written by the indexing model from the issue text.