Fix horizontal creation

[Darmody]

fix #353

1. always create new line after insert horizontal-rule
2. allows only paragraph and sub-list inside list

ref: #308

[codecov]

Codecov Report

Merging #455 (01e34cb) into main (8a01c97) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
+ Coverage   69.66%   69.68%   +0.01%     
==========================================
  Files        1060     1060              
  Lines       18027    18023       -4     
  Branches     2496     2495       -1     
==========================================
  Hits        12559    12559              
+ Misses       5155     5151       -4     
  Partials      313      313              

Flag	Coverage Δ
frontend-unit	65.33% <0.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
.../src/extensions/blocks/horizontalRule/inputRule.ts	31.25% <0.00%> (+6.25%)	⬆️
...-editor/src/extensions/blocks/listItem/listItem.ts	100.00% <ø> (ø)
...-editor/src/extensions/blocks/taskItem/taskItem.ts	100.00% <ø> (ø)

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[github-actions]

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