Bump version to 0.4.0, upgrade Scala 3.8.3 and sbt 1.12.8

Documents the doc-tag extension in README and adds the previously
undocumented strikethrough, taskListItems, extendedAutolinks,
footnotes, smartPunctuation, and attributes flags to the
configuration table.

remove unused TestTest scratch file

release v0.0.23: merge title branch, collapsible inline titles

Merge title and dev branches into stable. CollapsibleBlock title
is now List[Inline] for inline formatting support. Fix indented
code block whitespace and loose list rendering.