Markdown syntax highlighting handles nested emphasis incorrectly

Description of the problem or steps to reproduce

Start micro

Execute set filetype markdown

Enter the following in the buffer:

**nested *emphasis* works**
*nested **emphasis** works*
***even the simplest form is affected***

Observe that it's highlighted very incorrectly:

Note: Regular grammars are incapable of highlighting this correctly; this requires a CFG. I assume there might be further issues with MD highlighting because of this, but this is the only one I stumbled on during actual use so far.

Note that this is valid, unambiguous Markdown; Pandoc handles it correctly:

<p><strong>nested <em>emphasis</em> works</strong> <em>nested <strong>emphasis</strong> works</em> <strong><em>even the simplest form is affected</em></strong></p>

Specifications

Version: 2.0.10
Commit hash: b9763856
Compiled on August 07, 2021

GNOME Terminal.

Reproduce the problem in micro 2.0.10 by setting the filetype to Markdown and entering the three nested-emphasis examples from the issue. Then locate the Markdown syntax-highlighting implementation and inspect how nested emphasis is handled. Done means the examples render with the expected emphasis highlighting without regressing ordinary Markdown highlighting.