fix: improve toc heading parse

[benjamincanac]

Fixes table of contents generation with nested code in headings.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (dev@cc2110a). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             dev     #279   +/-   ##
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  Coverage       ?   90.45%           
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  Files          ?       15           
  Lines          ?      545           
  Branches       ?      115           
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  Hits           ?      493           
  Misses         ?       46           
  Partials       ?        6           

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