#### Fix strong emphasis format (#18010 by @yin1999)

This is a supplementary fix for #17143, where the original PR could not determine whether the contents on the left and right sides were both words.

<!-- prettier-ignore -->
```markdown
<!-- Input -->
1***2***3
1**_2_**3

<!-- Prettier stable -->
1**_2_**3
1**_2_**3

<!-- Prettier main -->
1***2***3
1***2***3
```