Non-deterministic output for html-in-md rendering

The output order changes if `--all-features` is passed in versus `--features=` even in the case of the same feature set.

```bash
git clone https://github.com/WebAssembly/wasi-cli.git

wit-bindgen markdown wit --html-in-md --all-features
git add

wit-bindgen markdown wit --html-in-md --features=cli-exit-with-code
# note diff
# there's only one feature in this package so the output should be the same
```