#### Fix Markdown emphasis near non-breaking space (#17424 by @rkdfx)

Underscore-style emphasis adjacent to a Unicode whitespace (e.g. non-breaking space) was being rewritten to asterisks, because the surrounding text was treated as a regular word.

<!-- prettier-ignore -->
```md
<!-- Input -->
- _REPORTED_ — message received;
- _NEEDSINFO_ — additional info needed;

<!-- Prettier stable -->
- *REPORTED* — message received;
- *NEEDSINFO* — additional info needed;

<!-- Prettier main -->
- _REPORTED_ — message received;
- _NEEDSINFO_ — additional info needed;
```
