Markdown / LLM output routinely uses short language labels (`ts`, `sh`,
`yml`, `py`, …) that the Notion API rejects because code.language is a
fixed enum. Previously every such fence hard-failed the whole PATCH with:
body.children[N].code.language should be "abap", ..., "typescript",
"yaml", or "java/c/c++/c#", instead was "ts".
Introduce `normalizeCodeLanguage`:
- passes through any value that's already in the Notion enum,
- maps ~50 common aliases (ts→typescript, sh→shell, yml→yaml, py→python,
cpp→c++, cs→c#, dockerfile→docker, md→markdown, proto→protobuf, …),
- for unknown values, falls back to 'plain text' and emits a one-line
stderr warning so a single unrecognized fence can't break a long doc.
Applied to both markdown parsing (`notion block append --file`) and the
`--lang` flag on inline append/insert, so CLI-typed aliases like
`--lang ts` also work.
Closes#22
傅洋