Make markdownToAdf(adfToMarkdown(doc)) deep-equal doc: textBreak, empty keys, -0, typed carry fences, code blocks of several text nodes

src/markdown/text-break.ts

@@ -0,0 +1,16 @@
+import type { AdfNode } from '../adf/document.ts'
+import { identicalMarks } from '../adf/document.ts'
+import { spellInlineLeafDirective } from './directive-syntax.ts'
+
+export const textBreakName = 'textBreak'
+
+export const textBreakSpelling = spellInlineLeafDirective(textBreakName, '')
+
+// Whether CommonMark reads the pair back as one text node, where neither rides the carry (spec/flavour.md, Inline nodes).
+export function readsAsOne(previous: AdfNode, node: AdfNode): boolean {
+  return spelledAsText(previous) && spelledAsText(node) && identicalMarks(previous, node)
+}
+
+function spelledAsText(node: AdfNode): boolean {
+  return node.type === 'text' && node.attrs === undefined && node.content === undefined && node.marks?.length !== 0
+}