fix: add support for breakline and divider

[penkzhou]

1. current convert method documentToHTML will transfer the empty line into empty p tag. This will make when render the markdown content into html ignore the empty p tag which will ignore all the break lines.
2. add support for the divider tag. it will transfer divider into hr tag.

[CLAassistant]

All committers have signed the CLA.

[LucasXu0]

@penkzhou Please include the tests.

[penkzhou]

@LucasXu0 just add the test for br tag.

[LucasXu0]

@penkzhou There're two tests failed.

[penkzhou]

@LucasXu0 has fixed the failed tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7d8147c) 76.46% compared to head (85835df) 76.36%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #690      +/-   ##
==========================================
- Coverage   76.46%   76.36%   -0.11%     
==========================================
  Files         295      296       +1     
  Lines       13309    13319      +10     
==========================================
- Hits        10177    10171       -6     
- Misses       3132     3148      +16     

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