Incorrect emphasis handling

[mity]

(Distilled from https://talk.commonmark.org/t/i-dont-understand-how-emphasis-is-parsed/3866)

Input:

*****Hello*world****

Actual Output:

<p>*****Hello<em>world</em>***</p>

Expected Output:

<p>**<em><strong>Hello<em>world</em></strong></em></p>

More detailed rationale can be found in this comment: https://talk.commonmark.org/t/i-dont-understand-how-emphasis-is-parsed/3866/8