12c: the block directive forms read and spell !adf: openers and closers

src/markdown/block-directive-forms.ts

@@ -0,0 +1,9 @@
+import { blockDirective } from '../adf/block-directives.ts'
+import { listBreakName } from './list-break.ts'
+
+export function blockDirectiveForm(name: string): 'container' | 'leaf' | undefined {
+  if (name === listBreakName) return 'leaf'
+  const directive = blockDirective(name)
+  if (directive === undefined) return undefined
+  return directive.contentModel === 'none' ? 'leaf' : 'container'
+}