feat: support single asterisk to italic

[Xazin]

Closes: AppFlowy-IO/AppFlowy#1059

[codecov]

Codecov Report

Merging #92 (47311c1) into main (0f303cb) will increase coverage by 0.07%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   86.79%   86.87%   +0.07%     
==========================================
  Files         124      125       +1     
  Lines        7322     7381      +59     
==========================================
+ Hits         6355     6412      +57     
- Misses        967      969       +2     

Impacted Files	Coverage Δ
...event_handlers/markdown_syntax_to_styled_text.dart	75.72% <76.92%> (+0.60%)	⬆️
...rvice/shortcut_event/built_in_shortcut_events.dart	98.23% <100.00%> (+0.06%)	⬆️

... and 8 files with indirect coverage changes

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[LucasXu0]

LGTM.