Part the source into adf/ and markdown/ ahead of the parser

src/markdown/backtick-runs.ts

@@ -0,0 +1,14 @@
+export function fencedCodeBlock(info: string, body: string): string {
+  const fence = '`'.repeat(Math.max(3, longestBacktickRun(body) + 1))
+  return body === '' ? `${fence}${info}\n${fence}` : `${fence}${info}\n${body}\n${fence}`
+}
+
+export function longestBacktickRun(text: string): number {
+  let longest = 0
+  let current = 0
+  for (const character of text) {
+    current = character === '`' ? current + 1 : 0
+    longest = Math.max(longest, current)
+  }
+  return longest
+}