Variable tabviewRuleConst

Block rule for [[tabview]]/[[tabs]] with [[tab]] children.

Parsing strategy:

1. Match BLOCK_OPEN + name "tabview" or "tabs" (case-insensitive).
2. Skip any trailing text/attributes on the opening tag (ignored).
3. Consume ]] and optional newline.
4. Repeatedly parse [[tab Label]]...[[/tab]] blocks via parseTab().
5. Whitespace and newlines between tabs are skipped; other content fails.
6. Consume [[/tabview]] or [[/tabs]].
7. If no tabs were found, fail the rule.
8. Emit a tab-view element containing the array of tab data.